Mac OS X: lack of a tiling window manager. at least 50% of my complaints would be solved by that one addition. 40% of my other issues would have to gut the Finder.
Rumor has it they're finally making windows resizable from any corner, too.
Of course, I shouldn't be shocked any more since Apple (and Jobs specifically) constantly play that game. I love OS X with a passion, but I can't help but chuckle at this point when they do it.