If Mojang beats Valve, then it's guaranteed they'll win the whole thing, no matter who they're up against in the finals. Valve gets far in these things because the Escapist is composed primarily of PC players, which is also why Mojang is getting so much support. If these same PC players vote for Mojang over Valve then that's essentially the end of the competition. Personally, I don't mind all that much if they win, Minecraft is an awesome game and this competition is a yearly popularity contest after all, and Mojang has been very, very popular this year.
I think Mojang is unlikely to win against Valve, but it certainly isn't inconceivable given the moment Mojang has and the fact that Valve doesn't have many recent releases.
However if Mojang defeats Valve I think they'll still be up against some very stiff competition. As a PC exclusive developer there are a ton of gamers out there who simply haven't played Minecraft and while The Escapist has a very large PC base there are plenty of console gamers there as well. If it comes down to Bioware or Bethesda (prominent developers with large console and PC fan bases) and Mojang I would not be surprised to see the PC vote split and the console vote going against Mojang.
Yeah. I'm actually surprised Blizzard aren't overwhelmingly popular in these contests (what with WoW, Starcraft, etc).
The first few years Turbine dominated MM and it was partially because they advertised on their MMO news feeds. Against a single player game, or one without this sort of internet presence, it was hard to compete against. I could be wrong but I don't believe Blizzard does this - and in fact from what I gather Turbine doesn't advertise it as heavily either now.