An unsigned type is sometimes a poor choice for a loop index. The compiler then cannot assume the loop will terminate, and generate worse code. Usually this doesn't matter -- until you discover it did.

Good point. Probably not a problem in practice for most containers, but yet another reason for why the STL (and C library, for that matter) should have used a signed size type from the beginning.

Yep, it's to match vector/array size_type and difference_type, although I tend to just use size_t and ptrdiff_t directly.