The first thing that sprang to my mind when I heard of Steam Greenlight was that it would be a Kickstarter for games. However it actually appears to be a voting platform. It asks the players to choose what Steam should publish, with the most-voted games getting the nod. Fine, but it's not going far enough.
Voting with a yes is different to voting with your wallet. It's easy because it involves no commitment, and so the decision to vote comes naturally. Funding, on the other hand, is a much more deliberate choice. You might idly vote for a game just because its blurb sounds fun, but actually not purchase it. To decide to purchase you'd need more, to believe in the game and its maker. This is what lean startup people call customer validation, and it separates the real marketing stories from the fake ones.
My hope remains that Steam will eventually get into crowdfunding. It would be wildly popular, and Valve has the back end support and delivery mechanism, as well as the pre-qualified audience. What's stopping them really?