I've decided to check this site out since it was tweeted by the great Jeff Atwood himself. The first question I saw was "Advice on making ruby code more ruby-like", which is the kind of question I see all the time on Stack Overflow.
So what is the difference between the questions on SO and the questions here? It seems to me there is enough overlap between the two that the difference is not clear, and I may not be alone in feeling this. So far this site seems redundant.
I'm quite sure that that question would get closed as "not a real question" on Stack Overflow, for these reasons:
Stack Overflow questions are supposed to be about specific problems. "What can be improved about this code?" is not a specific question.
They should contain the shortest runnable code sample which demonstrates that problem. Posting large code samples without any indication where the problem is, is usually frowned upon.
Stack Overflow questions should have specific answer where there is one clear best answer. The question "What can be improved about this code" has virtually an infinite number of distinct answers which are all equally valid.
Besides the difference in missions and allowable questions (Stack Overflow helps you fix non-working code; Code Review helps you improve already working code), the two communities have different practices as well.
Stack Overflow prefers generic questions of general interest. Code Review prefers to see your code just as you wrote it, with all details that might be relevant to the review.
On Code Review, everything about the posted code is reviewable. Therefore, the code in the question must be preserved verbatim, including aspects that have no effect on functionality, such as whitespace, naming, and comments.
Code Reviewer answers may focus on a different issue than what the question asks. For example, the question might ask about how to write comments on a complex regular expression, but one of the answers might point out a performance problem instead.
On Code Review, there may be no single right answer. Answers may disagree. It's even acceptable for one user to post multiple answers (focusing on different aspects of the code or proposing different solutions to an issue).
Don't expect an instant answer. Code Review works at a slower pace, both because we are a smaller community and because it takes time to write a good code review.
I agree there's a slight difference in focus, but they're close enough that I think separating them into separate sites would diminishe their audiences.
Just a personal take: I'm browsing Code Review today, but I'll probably rarely come back unless I have a really specific question that I somehow feel would get better answers here. I'm an active Stack Overflow user but I don't have time to increase the number of Stack Exchange sites I look at.