The main things I look for in a writer's group, in no particular order and just off the top of my head:
-People that are willing to give--and receive--real constructive criticism. If all someone has to say about a piece is "that was great," they probably didn't read it. Likewise, if someone only wants to hear about how awesome they are, they're wasting their time and everyone else's.
-Diversity. Whether it's people who are from a different background, or in a different place in their lives, or just plain write a different genre, I've found that exposure to perspectives far removed from my own has made me a better writer.
-On the flip side, I need people who get my genre, even if they don't write or read a lot of it themselves. If people are asking me why I'm not using Star Trek technology to resolve the plot of a hard-sf story, or looking down their nose at me for not being as "literary" as they are, it's not a good fit.
-Committed critters. The golden rule of crit groups is crit unto others as you would have them crit unto you. If people are all focus and attention when it's their turn to be workshopped, but off-topic or absent when it's time to return the favor, well, those aren't the kind of people I want to be investing my time in.
These points are pretty much all about people. I've found I can adapt to most group structures and crit systems if the people are worth it. But it's important to know what kind of time commitment you're making when you join a group, and whether or not their crit system is going to be helpful to you (do they go around in a circle and have everyone answer set questions, or is it more of a conversation? Is the person who's work is up for discussion encouraged to speak up or stay silent? Is everyone reading the same thing at once, or is work passed around more informally?).