How many times have you heard someone say "I'd use Linux for my desktop if it could run $GAME"?
This is the biggest reason why there hasn't been as much adoption -- a great many games are simply not yet ported, or their ports haven't been made ready for Steam distribution.
If a gamer's "workflow" is complicated even a little bit, such as requiring a reboot back into Windows to play one game, people will take the path of least resistance and just stay booted to Windows. This is what I currently do on my rMBP during heavy gaming sessions, because games like NBA 2K13, Skyrim, and Rift aren't yet supported in MacOS/Linux.
Once the top 50-100 games are ported, Linux desktop adoption will probably explode, correlated closely with the Steam market share held by Linux users, as the last bastion of excuses for using Windows finally fades away.
My recommendation for the day is Kentucky Route Zero. It runs beautifully on my mid-range Ubuntu laptop and frankly it has been one of the most astounding gaming experiences I've ever had.
