I recently posted a comment about why I've switched back from Mac OS X to GNU/Linux. I've been thinking about it a lot since I did it, and I'm pretty happy with the comment, so I include it here. Slightly edited, and will likely change over time.
I type this on a rev-a iMac G5 healthily running Debian with almost all the hardware fully supported, including the Airport Extreme wireless. It was a bastard to start doing, but from June 2005 having used Mac OS X for about six months (I got it in December 2004) till when I had it finally acceptable some time later, it became very important to me: The Mac OS X UI simply doesn't work for me.
The Finder keeps opening up in that stupid space-wasting sidebar brushed metal mode; it remembers my settings for old folders, but for new ones it falls back on its default. It uses file extensions to determine type: On Debian, I use ROX-Filer, which uses a file's contents to determine type whenever it can. I don't recall the Finder as having a way to create files from a set of templates in a menu: This means that if you're in a folder, and want to make a new file there, you have to (a) stop what you're doing (!!!); (b) go to the applications folder, (b ii) remember the name of the program you want to use, (b iii) find it, (b iv) and run it; (c) choose to save the new document; (d) find the folder you already have open once again in the save dialog box; (e) resume whatever you're doing. (This list, and any others I include, are based on how the process feels to me. So I get to include steps like "stop what you're doing". And any usability guru will tell you this means there's a serious problem!) It has other nasty habits.
I'm used to X11's selection clipboard, where middle-clicking pastes whatever's been selected, so I keep forgetting to choose to copy. Major cut to my productivity!
Windows are grouped by program, not by task. I might have one browser window open for work, and another for play; if I'm using Terminal and Preview also for work, I don't want to have to keep pushing the play browser window out of the way. (Actually, I found different programs behaved differently with regards to this. I can't remember which or why.) Relatedly, there's no decent virtual desktop programs out there. Or perhaps, because most programs don't expect to use virtual desktops, they don't behave properly when they're asked to.
There's only one menu for every program. This makes it really hard to keep track of where I am. Exposé makes it worse; if I'm busy with Camino, and use Expose to show my desktop, Camino's menu still shows up. So I go to the "View" menu and get really confused by why "Clean up selection" doesn't show up. Apparently this makes going to the menu faster for some particular use cases, but the way I operate, it doesn't help much; aside from the fact I prefer context menus (which help me associate actions with objects), I'm usually using more than one program to do a task, so using a menu becomes a multi-step process: (a) Work out which app is focussed; (b) If necessary, (b i) go find a window for the app whose menu I want to work on, (b ii) and click on it; (c) Use the menu.
Actually, it probably makes me sound stupid, but I find this whole business of "the focussed window" really hard to keep track of. On the Mac, it's the topmost window, it's the only one you can operate (most) widgets on (most of the time), it's the only one whose application menu is visible, the window itself looks different (brighter, more contrastive), and it's got a shadow round it. Only
Missing is heavily penalised. I mention above the problems with missing the window's edge by a pixel or two, but there's more. The Close, Minimise and Expand buttons are so small that I keep missing them. And then click again, thinking the computer just didn't hear me. And so the window minimises. Not so bad if I'm going for the minimise button, but usually I'm not... Another way is that clicking on the separator in a menu causes the menu to close. Without exception, every time I clicked the separator I was actually going for the option right above or below it. Considering the separator takes up an invisible amount of space above and below it (the line itself is only a few pixels, but the height that causes the menu to close is the whole size of a regular menu item), I found I could do this two or three times before I got the actual menu. This could be easily solved by doing what every other operating system does nowadays: Highlighting the menu option underneath the pointer.
The Dock sucks. In more ways than I can count. It's pretty, but confusing and stupid. It's divided up into two groups, but it holds five completely distinct things: Application shortcuts; running applications; document shortcuts; minimised windows; deleting stuff and ejecting media (leaving aside the question of why launching programs is different from opening documents anyway!). It's either invisible, or it's in the way. Even if there's plenty of stuff to work with next to it, it still insists on preventing you from making a window taking up all the rest of the vertical space from taking up the space next to it. (I've heard other complaints about parts of the dock, like the magnification. Actually, I love the magnification. I've run the Mac with it turned off, and I've found the Dock even harder to use.)
Oh, also, I couldn't get any decent program to play my collection of Ogg Vorbis files. I still don't think there's any decent program that can play Ogg Vorbis files. This is a real downer!