So, Greenlight launched and had some issues. An upvote and downvote system that didn't make a lot of sense (downvote? So you're opposed to a game being on Steam?), but the big problem was a massive flood of...well, crap. Some legitimate indie games were up there, but also lots of "I just completed the tutorials in Game Maker" stuff as well as suggestions for games that Greenlight isn't even for (ie, people were posting Half Life 3 wanting it on Steam....what?).
Valve has already made some changes. The voting system has been made more clear. Upvote if you would be interested in buying the game on Steam, and downvote was replaced with a "no thanks/not interested". I'm not sure what they do with the downvotes, but it might just be to clear that game off your big list of games you haven't voted on yet.
They also added a $100 fee to list a game on Greenlight, to cut down on the flood. This is interesting and has its own issues.
Giantbomb has a good article. Now, the $100 is per developer not per game, and the money goes to Childs Play, not Valve. But its a strange barrier. Sure it cuts down on the flood of submissions, but now you have to spend money just for the chance to get rated high enough for Valve to look it over and maybe decide to let you on Steam. To contrast, Apple has a $99/year fee for the app store and as long as you don't break their rules you're pretty much guaranteed to sell there. One question is why $100? If it was just to cut down on spam, why not $50 or $20?