Steam doesn't seem to have a large selection of games....
Yet. It's only be available for Mac for a month. There's over 1000 available for Windows, where Steam's been around for 3 years. Many of them are older games that run on DOS Box, so they should be easy to bring over to Mac.
Steam is *the* main content delivery network for Windows, so they already have distribution deals with pretty much everyone (well, not the Mac-specific porting companies like Aspyr, but other than that). New games for Mac will almost certainly be available on Steam as soon as they're released, and Valve (the company that makes Steam) has committed to Mac releases the same day as the Windows release on all their future games.
For now, most of the games available for Mac (and they're all Mac native, not ports like Aspyr's games or EA's games) are excellent. Everything Valve produces works well- just buy the Orange Box which contains the Half-Life 2 series (FPS mixed in with some puzzles), Portal (world's best puzzle game), and Team Fortress 2 (online multiplayer FPS). Also, get Torchlight (a Diablo-style dungeon crawling RPG) and World of Goo (an indie puzzle game).