Titanium-
The strength of titanium alloys is what makes the large, over-350cc, heads of modern golf possible. The larger heads and increased perimeter weighting have definite technical advantages for the average golfer, and even more for the struggling golfer.  The flip side is that the titanium drivers feel too light, even though they are the same weight as stainless or persimmon clubs. They also have very little feel on impact, kind of like hitting your golf ball with an empty pop can.  For those golfers who need feedback to maintain a good swing, titanium is not a good choice

Design-
You always swing a driver at a teed-up ball, and you usually want to hit it as far a possible, rather than a specific distance.  It is a single purpose club, and these requirements change the way the driver should be designed, and the driver that you should swing.  A driver designed to "look" like part of a matched fairway woods set... is a badly designed driver. For instance, sole shape is important on a fairway wood, while face shape is a lower priority... just the opposite in a driver. Wood "sets" work for the retailer, but not for you.  If you are a new golfer, buy your fairway woods first, develop your swing, and THEN go shopping for your driver.

Driver Length-
The length of your driver affects both distance and accuracy. Longer means potentially more distance, but always less accuracy.

It is not accurate to say that a longer shaft necessarily means more clubhead speed. Its true that, all other things being equal, a longer club will have more clubhead speed, but all other things are NOT equal, and in some cases a longer shaft can mean slower clubhead speed at impact, PLUS less accurate impact.

A standard modern men's off-the-rack driver with a fiber-epoxy shaft is 45" long.  That's an inch and a half too long for a "normal" male golfer who is 5'10" tall and has typical control over his swing path.  That's also a full inch longer than the typical driver shaft on the PGA Tour, where they average over 270 yards off the tee... you really gotta wonder about that!.  See "A Short History of Golf Club Sets"

43.5" is the clubmaker's standard for a full-weight steel shaft. For every inch you are taller or shorter, we will typically add or subtract 1/8", and then add or subtract fudge factors for your ability to hit the sweet spot, your hand-eye coordination, your stance, and your age.  You will be amazed at what a difference this makes.

Some fitters recommend adding a half-inch if you install super-lightweight shafts.  Not me. The extra five yards does not justify the significant loss of accuracy.  And you will lose the five yards anyhow if you average hit is just a 1/4' further off the sweet spot.  Furthermore, a lighter weight club is NOT easier to control... it is harder to control.

Shaft Stiffness-
The stiffness of the shaft probably has more effect than any other specification of the driver, to allow the golfer to hit consistently good drives.  The related specs of torque and bend profile are also important, but just a little bit too stiff or bendy of a shaft will make the most expensive driver incredibly hard to hit well.

The stiffness of the driver shaft has a big effect on the trajectory of your drives. Bendy shafts hit higher, stiff shafts hit lower.  A very small difference in stiffness makes a big difference in trajectory.

Technically a stiff shaft is best for every golfer, because they are inherently more consistent and don't compound your swing errors. However, in combination with your driver's face loft, the stiffness of the shaft can be used to fine tune your effective loft angle at impact. In combination with several other factors, this can optimize your launch angle, to maximize your driving distance.

More important, though, softer shafts "feel" better, and this can be an issue in you ability to maintain confidence and consistency from swing to swing, and improve over time.  This is more important than the purely technical engineering issues, so very few golfers actually need a "stiff" shaft.  Conversely, a shaft that is too soft for your abilities will feel mushy, floppy, and uncontrollable, which will degrade your swing right now. So the technical issues must be put aside, in favor of matching the shaft to your ability to feel its feedback, which is closely related to your swing speed.

If your wrist and elbow joints are getting old and creaky, a stiff-shafted driver can be hard on them. You can use a bendier shaft if you prefer, we just select a lower-lofted driver face to compensate. 

If you decide on a softer shaft and prefer glass shafts, we will have to spend some money on it; cheap ones can bend inconsistently, causing both accuracy and distance problems, particularly if you are a hard hitter. 

Head Size-
Driver heads now come in sizes up to 650cc, 300cc being "oversize", and 400cc being "big". 460cc is now the USGA maximum for "legal" competitive golf. Head weight is usually right around 200-205 grams, regardless of the volume.  The advantages of large face size and perimeter weighting for the average and high-handicap golfer are pretty well established; more distance with better accuracy. But it doesn't work for everybody, some people just can't hit a big head. And you should use a strong, low-tip-torque (expensive!) shaft if you swing a big head

For strength reasons, the big driver heads have to use hi-tech materials... anything over 330cc will be made of something stronger than stainless steel. The exotic and costly materials that make big drivers possible may be worth the money if you are looking for something extra off the tee.  The fact, however, is that some people still drive better with stainless steel, and steel head are very hard to find anymore, even in the component market.

Face Angle-
"Launch angle" (the vertical angle at which the ball leaves the tee) is the important thing, but its only partially caused by the loft angle of the club face. There is an ideal loft for your swing-speed, swing-path, and shaft stiffness, which is particularly important if you are a big hitter, but still of benefit to average golfers. Driver heads are available in lofts from 2 degrees to 14 degrees, sometimes in one-degree increments, so we can really tune this just for you. This is all about getting the ball off at the right launch angle for best total distance, and I can just about guarantee you that if you are not a 15 handicap or less, the 10.5 degree drivers in the retail shops will rob  you of distance.

A standard men's off-the-shelf driver for an average male golfer has a face angle ("loft") of 10.5 degrees, with 9 and 12 degree lofts being commonly available. A few driver models come in one-degree increments, starting as low as 6 degrees and going up to 16 degrees. Longer hitters do better with a lower face angle, and shorter hitters will benefit from a face as high as 18 degrees ( a 4-wood!). Stiffer shafts require weaker (numerically higher) lofts. 

Bulge and Roll-
This refers to the amount of curvature in your club face. It is put there on purpose to compensate for the twisting effects of slightly off-center hits. It also adds strength to the club face. Both of these specs are engineered by the club head designer to match the weight distribution of the head, and produce the optimum correction. There has been some research lately suggesting that non-circular and non-linear curvature would work better, and there are a few club heads now coming out to take advantage of this.

Bulge and roll are important specs on your woods, and the main reason you should not mess around with the weight distribution , even if your clubhead comes with "weight ports" intended to allow you to change the weighting.

Weight Distribution-
Many driver head designs try to make the face as thin as practical, and put more weight in the perimeter or back of the clubhead. The face of your driver has to withstand a 2000 pound impact, so its a bit of a challenge for the designer as he makes the face thinner. That's why all the exotic materials are useful in a driver; they allow a lighter, thinner, bigger face to be strong enough to withstand the impact with the ball. 

For best energy transfer, a driver head should weigh right around 200-205 grams. From one design to the next, there are differences in how much of the excess weight the designer puts in the heel and toe, or in the sole, or recently, at the back of the head body. This is a complex decision, because it affects the center of mass and the moment of inertia, and energy transfer, and face strength, and body deformity.  Sole weight gets the ball more airborne, which is helpful if you are not a long hitter.  Heel and toe weight will give you a slightly more accurate shot if you don't hit the sweet spot, as does a rear-ward center of mass.  On the other hand, more weight forward gives the club a more solid feel at impact, and feel is important.  Even worse, all that weight a the back, toe, heel and sole, make the club less workable, an important consideration for a good golfer.

Many modern drivers now come with adjustable weights.  Obviously, this flies in the face of what I just said about the importance of keeping the weight distribution optimized for the bulge and roll.  It also obviously affects the "sweet-spot".  I can guarantee you that changing the weight distribution will deliver the advertised effect of inducing more or less hook or slice... but you may not like the overall result in terms of distance and consistency, and an unbalanced club has lousy feel. 

Be careful about changing the  total weight of a golf club, too.  Its easy to do, just stick a little lead tape on it, and you've got a heavier club.  But you have changed the balance dramatically, and several other things too!  Most important, you have changed the center of mass, completely destroying the carefully calculated bulge and roll effects of the face.  

Never add weight to the sole of your driver. For instance, add an ounce of lead on the bottom, and you may like the powerful swing feel it produces, but you will not like the trajectory of your drives. It will move the sweet spot almost a 1/4" lower, producing worm-burner low balls when you hit the new sweet spot, and powerful but very short diving  balls when you hit the old sweet spot.  It can take 50 yards off a golfer's average distance.

Putting weight on the back of the club will exaggerate the carefully calculated gear effect of the designed-in bulge on the face, while reducing the club's workability... the worst of both worlds! Nonetheless, if you want to add weight to your club, this is the place.  Try to get the tape right behind the sweet spot, horizontally and vertically.  Experiment with it at the driving range, with a roll of lead tape handy, because it is tough to get it right.  And when you're done, ask yourself if you really like the overall result.

Coefficient of Restitution-
"COR" is one way of measuring the percentage of the impact energy that transfers from the clubface to the golf ball.  Ideally, you would want 100%, but 88% is the practical limit on it.  Golf's rule-making bodies have further restricted it to 83% for competition. However you can get lots of  "non-conforming" heads if you want a hotter club just for your fun rounds.  

In modern club-design idiom, "COR" has come to be equated to "spring face effect", which is really only one factor affecting COR. You have to hit the sweet spot (lateral and vertical center of mass on the club face) to get any advantage from a well-designed spring-faced driver, but it can be pretty impressive when you do.  Some folks claim that if you hit off-center, a spring-face driver will throw your ball further off line. Some designers also claim to have found ways to increase size of the sweet spot.

Nearly every clubhead now advertises that it has high-COR spring-face effect.  The really good ones are highly engineered, use exotic metals and do not come cheap. 

Overall Weight-
Your driver is the lightest club in your bag, usually between 198 and 206 grams. This is ideal for only the average-to-good hitter, but it is pretty close for almost everyone.  The head needs to be around 200 grams to get the best compromise between energy transfer to the 46 gram golf ball and a normal person's ability to accelerate the club head. The mass of the clubhead also affects the way the overall club interacts with gravity and the "double-lever" inertial effects that account for most of the velocity of the clubhead at the bottom. 

It is simply not feasible to build heads in a choice of weights. However, it is entirely possible to ADD weight to a clubhead if that would benefit your swing.  But be careful WHERE you add that weight. (see "Weight Distribution", above)

If you tend to cast off a lot at the top, a little extra weight added to the clubhead area may  get you considerably more distance and accuracy.  Experiment with this with care, though, and look at the weight of your shaft before changing the weight of your clubhead.

There are some benefits for many average-to-better golfers from having a lighter shaft on the driver, allowing more acceleration of the clubhead. This can be overstated, because the saved weight is in the part of the club closest to your hands, not in the end that has to be accelerated. Still, there is some advantage for the average to better golfer.  On the other hand, many  very good golfers prefer the standard 120 gram shaft and 88 gram grip, because they like the inertial feel of them and find lighter shafts harder to control. Beginning golfers should avoid lightweight shafts, and most would actually do better ADDING weight to their clubs as a training tool, to reduce their tendency to cast off.

Balance-
Golf clubs have a form of balance measured as "swingweight" along the length of the club, and a more subtle and hard-to-define balance around the shaft, and also a form of balance called the Moment of Inertia.  You can feel these but I have never seen a  meaningful explanation of the effect of them.  Nonetheless, you should check the balance of every club before you buy it.  You will know it when it feels right.