It's great news that Steam supports Linux, and will surely help with gaming on Linux. I've heard that Ubuntu is the distro that Steam officially supports, so it follows that you'd have the best Steam experience on Ubuntu.
My question is this:
Would any of the differences that make Lubuntu what it is and not Ubuntu, affect the quality of experience I would get with Steam?
The desktop environment that you use does not affect the usage of Steam. In fact, you do not even need a desktop at all to run steam.
An example of that is that a Steam session can be added to LightDM or any other login managers to run Steam in big picture mode instead of the actual desktop.
And by the way: SteamOS is built on top of the stable release branch of Debian, not Ubuntu, Ubuntu is as well officially supported as any other Linux distribution. Not that it matters, most of the popular Linux distribution will run Steam without any issues.