In a succinct way, how I feel about Greenlight is dubious. I've seen some other people making comparisons of Greenlight to Xbox Live Indie Games and from what I've seen being listed I can't help but agree. Similar to XBLIG, there will be a lot of terrible games to sift through if you want to find some of the good games that are being posted to it and a lot of those garbage games aren't people who are really into making that particular game that they're putting up, but they're putting that game up because it seems like what they're trying to do is cash in on whatever craze seems like it'll sell well. There are numerous Slenderman games up for voting, several Minecraft like games (in both visual and gameplay similarities), a fuckton of zombie games because that dead horse hasn't been beaten enough.
The saving grace of Greenlight is that unlike XBLIG these aren't all games that are immediately being put up for sale once they're finished. But the downside is that if you just look at whats popular on XBLIG is exactly that type of game, where no one has any creative ideas at all and they're just all jockeying for position in who can cram together the most profitable ideas to be the Next Big Indie Hit.
Obviously people who are putting the time into making games want some return out of them if they're bothering to put them up for sale rather than on some indie shareware/freeware site. And that's fine that they want to make money from their work. But they're going about it in the most cynical way of just combining popular things as a gimmick rather than focusing on making an interesting to play game, let alone a game that's creative and does something original and neat.
This is what I'm worried about with Greenlight. That the service will become mired down with these quick low effort shallow gimmick heavy cash grab games that have managed to be the proven success records on XBLIG. I'm sure that Steam will police what gets put up for sale more harshly than what Microsoft does with XBLIG, but in the end they're still a business out to make a profit. On the flip side they want their platform to still be appealing to larger publisher's who wouldn't want their big name games to be burred under poorly made indie stuff, so there is always the chance that Steam will be more rigorous with what gets added even if there's popular demand for it.
All of this said, I think this service has a lot of potential to bring a lot of smaller title neat and well made indie games to the attention of more people. TowerClimb, a roguelike-like game where you try to reach the top of a tower (very creative naming) has already been added by it's creator and it'd be neat to see that get some momentum. It'd be neat to see something like Escape Goat from XBLIG and Desura brought over to Steam, it was a really solid puzzle platformer. There are quite a few adventure games on the list that seem like they'd be pretty neat to play through, and Visual Novel games are being put up as well, though I can't really attest to the quality of any of them as I'm not a huge fan of the genre.
There's some neat stuff there for sure, but I'm just worried that the stuff that actually has some creativity and uniqueness to it will be buried under and avalanche of shit cash grab games.