Don't forget Macs. One of the best things from Valve was Mac support with the Source Engine. It would be ashamed if we Mac people couldn't use the puzzle creator.
If it is going to be "fully open" I'd like it to be available for other platforms. Mac and Linux, of course. They don't have to have the same games. For instance, Tomb Raider exists for the Mac and having Steam for the Mac would allow the publishers to expand their distribution.