I hate Mac OS X's GUI.  My credentials to rant about this are: over the last 15 years, i have used the following desktop operating systems at home, school/uni, or work (in alphabetical order):

• AIX 3.1, X11/Motif GUI
• HP-UX 7.x-11.x; X11/Motif, CDE GUIs
• Linux (kernel 0.97 - 2.4); X11, KDE, and GNOME GUIs
• MS-DOS 2.11 - 6.22
• Mac OS 7.x - 10.x
• Minix
• OS/2 1.3-2.1
• SunOS 4.x - 5.9; Sunview, Open Look, and CDE GUIs
• Windows 3.x, 9x, 200x, XP

My current desktop is Red Hat Linux 9, with GNOME 2.2 GUI, and icewm as my window manager.

In the last 18 months, i've been working in a new job and i've had to support Mac OS X at work.  Here are some of the observations i've made about the GUI design:

• Esc and Enter do not reliably map to Cancel and OK in dialogs.
• Maximise and Zoom (under 10.2) are not logically named (Maximise gives you a window smaller than the maximum, and Zoom doesn't actually zoom in or out.)
• Worse yet, true maximise functionality has been completely removed from Finder and Safari in 10.3.  In order to make full use of your screen (except for the dock), you have to manually move the window up to the top left of the screen and manually resize it to the maximum extent.  Most people (at least, most males) like to work on one thing at once, and it's only a rare occasion when we want to see more than one window.  (And even if we do, we probably want it to behave more like Windows' "Tile horizontally" or "Tile vertically" functions.)
• Dialogs do not layer on top of each other (instead they roll down and roll up), so there is no way to see the context when you have multiple levels of dialogs open.  Mozilla's preferences dialog is an excellent example of this.  (Some may argue that that's not the best way to arrange dialogs, and that may be so, but the point remains that if you open multiple dialogs, they should pop up on top of one another, not completely replace one another.)
• For those of us who like quiet to be able to concentrate, the above-mentioned roll down and roll up is visual noise.  At the end of a long day, it induces mild nausea (at least in me it does).  There should at least be an option to turn it off.
• The horizontal line pattern (just about everywhere in 10.2, reduced in frequency in 10.3) causes eye strain after just a few minutes.  After a couple of hours, you feel like you've got permanent stripes in front of your eyes.
• Some applications (notably anything running in the Classic box) don't know about the dock, and some do, so maximise behaviour (when it works) is not consistent.  Those applications that don't know about the dock overlap it, causing part of the application not to work.  Windows' task bar behaves much more sensibly: if you want it to auto show/hide, it sends maximised applications a resize event to make them fit maximally; if you don't, it stops applications from overlapping it (at least when it's at the bottom of the screen; if it's at the top sometimes this doesn't work flawlessly).
• The dock performs two functions: starting new programs and switching between running programs.  This is error prone because if you miss the icon you're aiming at slightly, you end up starting a new program instead of switching to one you had open already.  This is especially annoying on slower machines (e.g. 400 MHz PowerBook G3).  Windows' separation of the task bar and the start menu is much easier to use.

Mac users, please don't be offended by all this - it's not your fault.  I just think that Apple made some fundamental GUI design blunders, and when folks rave about the interface, they're being bamboozled by the flashing lights and bright colours, and not paying attention to certain issues that have a marked effect on productivity.  Nearly every one of the above GUI facets is much better on Windows than Mac OS X (not that i recommend it, since it's much less reliable than OS X).  Even Mac OS 9 had a better interface than OS X.

For the record, here are the things i like about Mac OS X:

• Stability: it crashes an order of magnitude less often than Windows
• Security: get a good virus scanner (i've found Sophos Antivirus to be quite good) and update it regularly, and enable your firewall, and you can just about forget about having your system compromised by rogue code
• Multiple monitors: they are a piece of cake to set up and very easy to use.
• Modem dialling: the ability to dial from a little icon in the menu bar is great.
• Network preferences: having multiple profiles is nice, although it could still be improved (for example, by remembering the IP address last given out on each site, and automatically selecting a profile based upon which one matches).

The first two of the above are critical for me, and are a good reason for most people to switch from Windows, in my opinion.  However, modern distributions of Linux have a more usable interface for prolonged use by experienced computer users, and are just as strong on the stability and security front.  Thus, you might hear me refer to Mac OS X sometimes as "poor man's Linux".  ;-)