The prime function of any race-specific product is to perform the duty of it’s training-specific equivalent. When a wheel fails in this regard you’d have been better off using the cheaper, heavier, less aerodynamic version. So, first off, is the wheel strong, durable, and sufficiently robust to do its duties? What are these basic duties?
The hubs must function appropriately. The wheels should spin freely, with no “indexing,” that is, you shouldn’t feel any hump-hump-hump of the bearings as they move with difficulty against their surfaces. At the same time, there shouldn’t be any play in the wheel as you move the rim back and forth after it’s mounted on the bike. That would signal that the bearings are not snugly seated against their surfaces.
The cassette assembly needs to be robust and its constituent parts strong. Don’t worry about the noise your cassette makes when freewheeling. Some of the noisiest hubs, like those made by Chris King, are also some of the best. Lots of noise means lots of pawls (they engage the ratchet ring and allow for your forward propulsion) and a crisply operating mechanism. Also don’t worry about how long your wheels rotate in the air after giving them a roulette wheel spin. This means next to nothing and is not the way to judge the performance of wheels.
Your wheels must be relatively true and stay true. Don’t expect discs and tri spokes to be as round as metal spoked wheels. They probably won’t be. If they exhibit “runout” (total side to side movement) that is within one millimeter, your wheels are fine and will perform their function. One nice element of solid wheels, beyond their aerodynamic advantage, is that they aren’t going to go out of true. Since there are more stressors applied to a bike’s rear wheel, a solid (disc or tri/quad spoke) wheel has the elegant ancillary benefit of staying perpetually within that 1mm runout window. Wire-spoked allow wheels can (especially if they're not made well) eventually become wobbly, or lose their roundness, and worst of all they can break. All of these effect performance, and as stated above the worst scenario is to race on wheels that don’t perform their basic functions on race day.
Because of all this it’s a good idea to consider wheels from companies who’ve been doing this a good long while. The reasons are legion. Deep carbon rims are hard to spoke up, and the tension on wire spokes can be very high and/or uneven. This transfers the stress to the spoke’s origin at the hub. If the hubs are standard and the spoke has an “elbow” this is a high stress area. Is the wheel company using its own hubs? Are the hub holes properly angled and chamfered so as to reduce stress in the elbow area? Is there enough meat on the flanges around the holes? Is the cassette assembly strong and durable? Are the bearing placements and surfaces properly thought out? Only experienced wheel makers are certain of having worked through these problems.
Likewise, with straight-pull spokes, are the hubs made of a durable aluminum alloy? Are the holes drilled at their proper angles? I had a chance meeting with Lance Armstrong last week, and asked jokingly if he was still mad at the wheel that blew up in Las Vegas. He laughed and said he didn’t remember the incident. I remember it clearly, as the front wheel on a bike of ours disintegrated during a triathlon national championship because we used spokes that did not penetrate the nipples far enough. It’s the only mechanical failure ever on a bike I prepared for a pro athlete, a career so far spanning 15 years. But what a failure and what an athlete on which to have it occur!
Then there are the braking surfaces to consider. Are they smooth, round and straight? Are there special brake pads required? Does the manufacturer supply them? Has this manufacturer enough experience with carbon surfaces to understand their nature, whether they brake properly when wet, or when hot? It's another reason to consider companies who do race wheels for a living, not as an afterthought.
All these are functions that any wheel ought to perform, and it's unfortunate when race wheels don’t meet these minimum standards.
Assuming basic functionality, lightness is near the bottom of the list of my important race wheel criteria. At the top of the list is aerodynamics. This is generally a function of shape and surface area. The more area on the wheel, the more aerodynamic the wheel. The difficulty is in realizing that the front wheel is more aerodynamically important than the rear, yet is also more prone to the handling issues surface area gives you. How do you know what you can handle? Depends on your size, and on the terrain and conditions you can expect in your racing. The smaller you are, the more sidewinds and buffeting will effect you. Are you intending to race on courses with fast descents? Things happen at 45mph that don’t happen at 20mph.
Finally, I find that these wheels require forks with more strength. Wheels have become so light that even a tire with a long-valve stem can create an unbalanced wheel, one that might be prone to a bit of shimmy under a bike with bad weight displacement. I am therefore not a fan of the lightest wheels, and also not of especially lightweight or radically aero forks. I'm more likely to ride a more standard fork and rely on its robustness to keep the wheel honest.
One postscript. When choosing your wheel, consider how it's inflated. Your style of floor pump and your on-board inflation system, must be compatible.
