Stack Exchange has worked in a few other areas (math, for instance), but on the whole, I feel like most of the sites are nowhere close to the level of expertise and penetration into the community for a given topic as Stack Overflow. Stack Overflow is one of the best programming Q&A sites; most programmers these days will come across it doing Google searches for questions they have, there are a lot of very knowledgable people there, and so on. I feel like a lot of the Stack Exchanges don't have anything like that level or penetration; they are not well established in their communities, instead they are places for programmers and sysadmins who are familiar with Stack Overflow to talk about topics they are interested in, without actually attracting the expert community that makes some of the sites work so well.
Part of the problem, I think, is that some topics are just much more suited to objective questions and answers than others. In programming, math, system administration, and so on, there are enough objectively right or wrong answers to keep subjective discussion from overwhelming the site. In other topics, what people really want is subjective discussion, polls, and so on, which just don't fit the Stack Exchange model well.
I wouldn't quite say that Stack Exchange has been successful in its expansion. It's had a few successes in other domains, and limited success in a few more, but there are a lot of communities that are really struggling to find their footing.