Little bit too short of a topic because of restrictions, but basically: why is Mac OS X always considered undeniably a (near-)perfect UI or operating system in general?
I'm sorry, but this isn't just out of ignorance of never using a Mac -- I had started to use one, though not full time, a few months ago with version 10.7. What I saw is a land of horrible complexity even compared to Windows, much less Linux. IMO Linux has far outshined everything Mac OS X does:
1. The window management is stupid. As an example: Apple is allergic to secondary mouse buttons so you have to do things like click the titlebar followed by cmd- to switch it to another workspace; it's a non-intuitive gesture that makes no sense in any context other than pretending mice still have only a single button.
(minor bitching point: the shadows on OS X are grossly overdone. Makes Windows Vista look traditional and conservative in comparison)
2. No good package management. Linux blows OS X far out of the water here; there's no scurry around trying to figure out where things installed too, and Mac OS X carries a ton of dependency hell, seriously.
3. Open source apps don't usually have precompiled Mac OS X versions -- this one actually surprised me. You seem to be expected by most projects to compile it yourself, which is error-prone and doesn't often work. Compare to Linux distributions providing everything under the sun pre-compiled.
3a. Just as an extension, I never got Wine to work. I thought it would be easy enough, even the home page says "Run Windows applications on Linux, BSD, Solaris, and Mac OS X" -- of course that last one is the only one that doesn't come precompiled. It is easier to run Wine on Solaris than ****ing Mac OS X; something is seriously wrong there.
4. Docks blow. I always had the feeling of hatred towards docks based on Windows/Linux implementations. I had some small hope that maybe Apple did it right as the chanted mantra goes. Nope, in fact it was much worse than some of the others I've tried, and I hated those too. Give me a Win95-style task bar any day.
There are more points I could probably put out, but maybe I just failed to drink the Mac OS X koolaid like I was supposed to. Everything about the operating system seemed antithetical to actually using the computer. It felt like I wasn't actually expected to treat it like a computer and instead treat it like a dumb box to play music and movies from; fuck that noise, I can do that on Linux without compromising my desire to run applications and use sane window management.