One thing to keep in mind about AA is that as great as it is, it is one tough hand to improve on. So, technically, its prone to become a weaker hand with each additional card (even heads up).

Also, an imporntant point to remember is that many times in heads-up up play one or both of you held the blinds. That means slow playing brings all kinds of two-pair garbage into play. As another poster noted, worst part is u can't even put a read on these kind of hands.