You might not actually know this, but, Microsoft did indeed make a Game for Windows Live Marketplace not too long ago. I can’t imagine it’s doing very well having to compete with platforms such as Steam. Even worse is that there is no way to use it unless you download the client. Without a way to force players to get the client like Valve did with Half-Life 2 and Steam it’s hard to get any sort of penetration into the market. Smartly in what seems to be an effort to remedy this next month you will be able to download and buy games directly through their website like EA and Blizzard.
This move will hopefully help them out with their Game for Window’s branding, but I don’t really think they can save that ship.