I'm fairly sure everyone is missing the main point as to why Valve and Blizzard are worried about Windows 8.
sure, the interface is an issue, but I doubt it's the thing these big developers are complaining about.
from the sounds of it their issue is with the POSSIBILITY of Microsoft moving everything to an app-store-esque set up. that is, everything installed on the computer has to go through that store. this closes down the system and costs the developer money (not just redeveloping, but paying Microsoft to have their application in the store).
it could be argued that this setup would even be a direct competitor to Valve (as someone in here has actually mentioned), which would cause them to worry. why would game developers go to Steam when they have a fairly equivalent distribution network on all Windows 8 machines instead of only the ones with Steam?
the thing is this hasn't been implemented, but it's not a large stretch to see it happening. Windows 8 is a tablet OS and app-stores are natural on tablets.
in any case, this seems to have caused Valve to start looking at supporting Linux, which is great. it will take some fairly large events for me to ever switch to Linux as my main OS, but it's still good to see it happening.