Valve is really a class act. They're using their money to open up gaming to an audience starved for a gaming fix. I mean, there are some decent open source games available for Linux, and Wine has really come a long way in terms of supporting Windows applications, but there has never been a solid presence of games for Linux. Using most Linux distros feels more fresh and rewarding than using Windows. They're very lightweight, they have support for basically all the applications and equivalents that I'll ever use, and there is so much variety to choose from. You can go with essentially pre-built configurations (I personally use Linux Mint 13 with Cinnamon), or you can take the leap and create something all your own with ArchLinux. Along with this, many things are very customizable, if you're willing to put in the time and effort to learn, and if you need help, many of the Linux communities are VERY knowledgeable and willing to help. To be honest, I'd be a full-time Linux user if it weren't for the lack of Steam. It seems that things may start to turn around now :)
Edit: Oh, forgot to mention one downside (depending on who you are): No DirectX 8.1 support since it's running off of OpenGL. I'm gonna kinda miss the ugliness :P