Look for DOCSIS compliance, preferably 1.1 or higher. This is a standard that allows generic cable modems (and related equipment) to interoperate properly.
Bandwidth isn't such a problem with cable modems, but latency and failure rate are rather more serious. They're especially serious as cable modems have become dirt-cheap commodity items; companies like 3com have stopped producing home-oriented cable modems because of the price wars. As a result, there aren't many high-quality home-oriented cable modems left. =(
Some of the worst cable modems I've seen are Motorola Surfboards. Some aren't completely DOCSIS compliant (this is part of why they're so comparatively easy to uncap), their latency is horrible, and they die more frequently than I like. Sadly, that's the cable modem AT&T is leasing to my house. The first one exhibited problems from day one, then failed after a few months. The second one (our current modem) has been working for maybe two or three months, and it's starting to get flaky (i.e. it occasionally takes a few hours to block sync).
Some of the best home-oriented cable modems available were the 3com tailfin modems (model 3CR2922x). They featured excellent speed/latency, very good reliability, DOCSIS 1.1 in hardware, a very cool sleek-looking design, and a nice little network-accessible diagnostics page. Although 3com has discontinued this line of modems, they still support it for those who already have one, and the modems are very reliable. They're not on PriceWatch too much anymore, but you can often find one on eBay.