What I like least about it is first metacity (yes, yes not bluecurve but GNOME2, but anyway), and the metacity theme that they did. This thing with rounded corners that's been wandering about in the Real World for some years and now have reached the desktop managers (Win XP, bluecurve, etc...) is really not for. It does look a bit more ... can't find the word in english ("fjolligt", as I'd say in Swedish, can someone translate it?) ... which I do understand makes a lot of people like it, but I really don't. It's not nearly as crisp as the old rectangular themes. I like most parts of the bluecurve gtk+ theme, though.
The thing that I don't like about metacity is that it's not half as configurable as sawfish is, and configurability is much of makes up a unix system. (That's why I'm really thrilled over the Hurd)