These notes apply to a single online critique group, which I tried out of a sense of curiosity. I did receive some very helpful comments, but overall, I decided that this group didn't work for me.
Mix of genres. This was great in one sense, because I was free to read works from any genre. However, it was awkward if I critiqued someone's children's story and the group's expectation was for her to read and remark upon my erotica. Some people did write in both genres, but that's not a reasonable assumption. I think some level of focus is desirable, so that everyone has some common ground. While there was an erotica group, it wasn't particularly large, and most of it seemed to be contemporary.
Reciprocal critiques. Members were actively encouraged to give critiques to those who had given unto them, which was great in increasing activity, but it meant looking at works for which you were completely the wrong audience for (see above), or working with writers who were at a very different stage of their writing development—which hardly works out to be equitable. I would sometimes spend hours on a critique trying to articulate the issues I saw in a work, only to get a 300-word (the minimum critique length) list of sentences in which I used the word "was" in return.
Inline critiques. One popular feature was the ability to insert comments after specific paragraphs of the story. I found that this encouraged what I call reader reactions: someone saying, "I guessed it was him!" or "I wonder what she's hiding?" This is great feedback, since ultimately I do write for readers; but the whole point of a critique group is that the other people are fellow writers who should be able to provide actual constructive criticism, and not a litany of which points in the plot were surprising.
Incorrect corrections. Frankly, I was far stronger in grammar than most of my critiquers. Too many people have read that the passive voice is bad, and concluded that any occurrence of "was" constitutes passive voice. I was hounded to change many sentences to use active verbs, although that would have changed my meaning. (Read Passive Voice and the Verb "To Be" [imogenhowson.com] for an explanation.)
Time commitment. As I mentioned, I would spend a lot of time on critiques at first: re-reading the stories, writing up comments at each pass, and trying to summarize major strengths and weakesses (or as my company insists on saying, "areas of development"). Critiques could go over a thousands words easily, and then I would actually edit this stuff. When my wrists started giving out, so did my standards, which made me unhappy. I simply couldn't afford to type anything that wasn't work-related or my own stories.
I'm sure many others have found this group helpful, and as I mentioned, it wasn't a loss for me. But the cons outweighed the pros, and I'm striking out alone again.