1. Good players.
It matters naught if your uber-GM has the game to die for if all the players do is argue, arse around or just even fail to engage. The first two are annoying but the latter is the most soul destroying. Given that in most games I play the GM is pretty clear up front what it's all about then there is no excuse. Turn up and tune in. And don't hog the spotlight either!
2. Space for self-expression.
I don't mind if there's a plot, as long as the players can do cool stuff too. And by cool stuff, I mean creating characters that aren't just always extensions of their 21st century selfs but people who can make stupid decisions for good reasons, be noble, fail, triumph, be spiteful, be lame or even self-sacrificing.
And I don't mean combat. Combat is undemanding conflict at best. Sure there are strategic decisions to be made but if the end result is PC death then it's a game killer. I want real choices with difficult decisions that are rather more engaging than the black and white offered by most combat. Moral decisions are the obvious choice, but there are also emotional issues such as love and loyalty than can be explored. Mystery too is fine, but works better for me if the crux is not a "spot hidden" roll.
4. No levels.
Well, not just levels, but 'advancement' in general that focuses on the mechanical aspects of the character. If my PC starts an interesting sub-plot about commercial involvement with a local hood, it won't matter much when I get two more levels and can push him around all I like. Focus on hard mechanics tends to overshadow softer developments because they destroy the (power) relationships in the game world. Even our weekly dishing out of GURPS xps is starting to get me down.
So, G, N, or S?