I personally dislike Greenlight because it means a game has to generate huge amounts of hype to sell. This is exactly the opposite of how I'd want to release a game.
If I'm not mistaken, I think the reason Minecraft isn't on Steam is because Notch doesn't want it on Steam.
Because you can sell more?
If I recall, Mojang decided a while back that a) it would be too difficult to integrate steam into their existing system and b) they wanted to do their own community (friendlists etc) thing (part of the reason to transition to general Mojang accounts)
All new games (or mods) should release a demo, because actual game-play is better than any trailer or video. That way you get a fair feeling about the game and can decide whether you want to buy it or not. The only downside is that we would maybe buy less games. Which is maybe why we don't see many demos for big games...
I did already voted that shit up bro.