SteamOS = Linux, so more devtools that run on Linux means more developers developing for SteamOS, which means SteamOS gains more traction quicker. So I'd vote for it even if I'm not going to be an early adopter.
Unity already lets you build for Linux. Develop on Windows / OSX, build for Linux. You just can't run the actual development tools on Linux.
We want the Unity Editor not just the Unity Player.
Debugging platform-specific issues without a debugger is less than ideal.
Unity lets you do remote debugging. Just build with Development + Script Debugging turned on and then you can connect your debugger to your game running on the same network.
Exactly. Linux is my primary OS and I'm not a game developer but I still log on Windows from time to time just to play with Unity and make simple games.