Why only Nvidia? Intel improved their open source drivers thanks to Valve, and AMD pushed some updates related to Steam as well.
And, while Steam officially support only Ubuntu (at least for now), Valve doesn't do anything for stopping it to run on other distro. They even modified the license to explicitly allow repacking, so that others could put the client in their repo.
They host on their site a .tar.gz with the Steam installer for distro that don't support .deb too.
Different Linux distributions are more binary compatible than BSD/OSX is.
Besides Steam has been repackaged for other distros but their official support is only for Ubuntu currently.
If they did start supporting other distributions it is difficult to know where that should end. Perhaps there is a case for supporting Fedora and perhaps Debian, but what about Slackware,Gentoo,Mint,Arch etc?
It would be impossible to guarantee it would work on every wacky distro out there.
Working pretty great for me on my Ubuntu+AMD desktop. Laptop is a clusterfuck of drivers though, Linux switchable graphics support sucks :(
Edit: Though I haven't tried anything too fancy yet, just FTL and Bastion. So maybe 3D gaming might not be as good as Windows but that's mostly fine for me for now. I'm fine booting into Windows to get my Starcraft 2 fix.
There are already packages for Arch, and as per the new licensing terms for the client there's no reason it can't or won't be packaged for other distros. AMD also improved their drivers with Valve. Not sure where the hate is coming from.
How much time are you giving it? Loading maps in TF2 is a notoriously bad experience; it is normal for the main thread to block for a long time (sometimes this is on the order of 20 seconds, sometimes 3 or 4 minutes in some cases).
Arch here, works fine. I had this issue for a 2 day period and did report it but it seems to have been fixed since then. You might want to try again.
There are many bug reports from non-Ubuntu users on GitHub and they all seem to get first-class support. Valve is pretty good about helping with that kind of stuff, they've even helped debug a few WINE bugs.
So, I normally have issues with fullscreen applications - in Windows and Wine. Fullscreen in Linux hasn't been an issue, but I'm pretty sure that's because the Valve/Source games run in "Windowed Fullscreen" so the window manager still gets to control it when you've released the mouse (ie, pressed Esc and are at the menu).
tl;dr: Try adding '-windowed' to the launch options for TF2 and see if that helps at all. (Steam -> Games -> Right Click "Team Fortress 2" -> Launcher Options and add "-windowed" minus the quotes obviously.
(edit: For the original thread, it also works decently well on my Macbook Air with Intel graphics)
13.04 on both of machines, though it was working last week on 12.10 before I rebuilt my machines.