This is just off the top my my head. Some of these might be plain wrong since I haven't spent that much time with the newest versions..
- General lack of configureability. I'm used to things like fluxbox and kde where "everything" can be configured. (Don't knock it til you try it ;) Especially keyboard shortcuts for launching stuff, expanding windows, minimizing windows etc..
- The window handling is terrible. Really, it is.. Can't maximize? Can't easily switch between two firefox windows? I used to think it was because I didn't know how to do it, so I asked and watched a few OSX users. But they just move windows around. I'm not going to handle 15 windows like that. Even if osx finally got virtual desktops (?).
- Better terminal (both the gnome one and the kde one are so far ahead, at least at first look)
- Unix/linux CLI programs that I've gotten used to being there by default. Also, I'm sure there are programs I use that simply doesn't run on OSX (not really experinced it, but it seems likely ;)
Also, the broader issue of customizability. Apple does a lot of things "right", but not everything. And while the "walled garden" do bring some advantages, its restrictiveness is a real pain when you're used to choose. It's impossible to do one size fits all in computing, you end up with something compromised, more tailored towards computer users quite different than myself.
FFM was never an option in the OS X window manage, outside Terminal.app, as it was fundamentally incompatible with the Classic event model, and I presume Carbon as well. There are some third-party hacks which attempt an imperfect emulation.