It seems at this point there's pretty much a limit on what can be done originality-wise for OSX themes.

I don't mean in terms of creating K-scope monstrosities, but little touches like moving/altering window buttons, using horizontal gradients, window borders, etc. are not (yet?) possible AFAIK.

Then there is the general concept that there's only so many variations possible in a long thin strip with three buttons... also, similarities are probably not an accident -- people are most comfortable with what they know, and I think there probably *is* a "sweet spot" in terms of UI design (balance btwn unobtrusive/distinct, readability, etc.) within the given (by Apple) constraints, that most popular themes seem to hover around.