Even if all games did run on *nix, the age of desktop Linux would still be a way away. Somehow (for reasons I can't entirely fathom) Linux continues to provide user interface that feels distinctly uncomfortable and even more inconsistent than Windows (that takes some doing, in my opinion). Few 'users' will choose to use Linux until someone 'does a Firefox' on one of the major desktop managers and cleans up the feature bloat and gets things tidied up together.
Normal people use Firefox because it's well designed. Normal people don't use the Mozilla Suite because it's a bloated with features that three people have ever used. Desktop Linux is in exactly the same situation as Mozilla was before Firefox: In need of a kick up the backside from a group of people who really understand software as a whole. Sadly, "Desktop Linux" is rather larger than the Mozilla software, not to mention rather fragmented in terms of separate projects who would all need the same culture shock treatment.
I don't see it happening any time soon at all, certainly not a scale large enough to produce a Linux distro that's consistent enough across the board to be tolerable. And it's not happening yet: Not enough talented UI developers are into Linux (most of the ones who want Unix are just moving to the Mac) and a distressing number of Linux hackers seem to regard UI with some kind of "UI is for n00bs" contempt and assume that 'usability' can be achieved by providing a check box for every inane command line argument they've come up with. Not enough people understand that designing good UI shares importance with writing low-level code when trying to produce a usable product.
Gnome have a chance in a few years, not least because they're working a little closer with Mozilla (unlike KDE who seem to assume that adding even more bouncy shite than Windows has will make it appealing). Maybe once Cairo (kinda link Quartz on the mac) is finished Gnome will leap on in terms of UI technology and tempt some better designers in to make a more accessible system.