Steam is the shit on PC, so it only makes sense that Steam on Mac would be even more the shit. Let's call 1-800-Steamer y'all.
Seriously though, this is some extremely good news. With the release expected sometime in April and GDC right around the corner there's plenty to be excited about. Some of Valve's best games are making the cut. Left 4 Dead 2, Team Fortress 2, the Half-Life series, Portal and it's long awaited sequel, Portal 2 are all expected day and date with the release of the Mac client.
All the features from the PC version will be available as well as cross-platform gameplay. PC and Mac users will be sharing the same servers and lobbies. The most profound feature is without a doubt, Steam Play, which allows users who've bought a game on any platform to download and play the same game on the other platform for no additional charge. That's what Steam is all about. Aside from a few publishers with shitty DRM practices, I'm looking at you UBISOFT, Steam is THE way to purchase games on PC. You can take your account anywhere, to any PC and play the games you've bought. With games like Modern Warfare 2 and Torchlight fully supporting Steam Cloud, even you're game saves transfer over. It's obvious that not every game will have a Mac counter part but even with what Valve has to offer that's more than enough to put a gigantic smile on many faces. Did I mention PORTAL 2!!!!
I personally can't wait. I'll definitely be jumping into more Team Fortress 2 as well as finally finishing up Half Life 2: Episode 2 now that Boot Camp is slowing becoming a thing of the past. Expect an update once the Mac Client hits.