Window's Alt+Tab also causes the Z-ordering of the windows to be most-recently-used. OS X's Cmd+Tab will surface all windows of a particular application to the top of the stack in their relative order, so if you have say, two browser windows, one you want and one that's been backgrounded with some long running train of thought, that (currently irrelevant) window is surface over the rest of whatever you're working on. Often I find this just gets in the way.

The Windows model more closely matches what I as a user want; when I'm switching, I have a particular window in mind, not any window, not all windows. Alt+Tab makes that more easily accomplished than Cmd+Tab/Cmd+`.

OS X's WM also seems to have an issue where, for whatever reason, when I Cmd+Tab to an application, it'll choose not the last active window, but one window in particular, even if that window is on a different, not focused workspace and there are windows from that application in this workspace. For whatever reason, it really likes my personal Chrome profile at work, at will switch to it at all costs, which is Grade A annoying during a presentation.