One is Steam on linux period. Even if it were its own little universe, having the support of Valve and the steam platform opens up a lot of opportunities for gaming on linux.
Another is Valve's core games on linux, which would definitely give a big kickstart to linux as a gaming platform.
Perhaps the most important could be when the top tier multi-platform game engines begin including linux as a targeted platform as a matter of routine.
Sometimes, mac ports that are done by third parties are not on steam because they can't split the revenue, so you still have to get the mac version separately.
But even with all the hurdles, I think this is a great step in the right direction.
Steam on Linux will probably ship with the audio/graphics/input libraries it needs, so 3rd party developers don't have to worry about environment nearly as much.
Wasteland 2 chose Unity engine because it's so cross-platform. It even has unpolished Linux support. The Linux port of Unity hasn't been released, because developers were doing that in their spare time and it wasn't up to Unity creators' standards. Now, InExile has been given Unity source for purposes of creating a decent Linux port. It's a single anecdote, but it illustrates what I'm saying above. Makers of Unity will probably want to get the most out of it, so Linux port of Unity will be contributed "upstream" and it's very likely that Unity will support Linux from now on.
I do not see Valve releasing clients for every distro out there.
For Linux/Ubuntu to be really strong in gaming, Steam is a big plus, however we also need strong driver support from nVidia and ATI. As far as I know, the best drivers on Linux are still not on par with their equivalent on Windows in terms of performance. If playing the same game on Linux means losing 10 fps on your favorite game, this may not be a very enticing alternative if you have both systems installed.
http://www.anandtech.com/show/5831/amd-trinity-review-a10-46...
The thing is, somebody updates the GPU drivers on Linux, with OS X most of the time you're tied to the drivers that came with your OS installation.
Forget performance, ATI drivers are so unstable it's barely usable (eg. Gnome 3 restarting regularly, like once per hour), and that's just plain desktop apps (Vim/browser), trying to run some version of VLC from default repo instantly crashed the PC (needed to restart), when I tried to play games random graphics bugs were everywhere (texture artifacts, framebuffer not clearing properly, etc.) All distros I've tried so far have problems with closed source drivers, and the OSS ones don't have HW 3D I think because it's slow as hell. Also - you can't alt-tab from full screen games. I looked it up, found a post that started with "you need to modify your xconfig" closed the tab. I think linux needs a lot more polish to be viable for everyday use as multimedia/gaming desktop OS, I would love to ditch the windows partition and rebooting but I don't think it's going to happen soon.
I guess that would depend on how the client software is built and if they utilize the shared libraries included in the Linux distribution. If dependencies are compiled into the distributed binaries, it's really not that hard to repackage the artifacts into different packages for different systems.