I don't expect 10.5 to be much better, but I hope I'm surprised. Of course OS X works very well for me, but that's not the point. Here's the list, in no particular order:
1. It’s not hard to put an icon behind the dock, esp. when the auto-arrange option is set for the Finder. If the user can’t control the doc (it's been locked as part of system configuration), the icon may not appear to be retrievable.You do get the feeling nobody in Cupertino cares about this stuff any more ...
2. It’s too easy for users to drag things and make them go “poof” -- with no obvious way to undo this.
3. The options to customize toolbars is too limited. Where’s the Help button?
4. There is NO documentation on how to use OS X in the box or on the machine. NADA. You need to buy a book.
5. The trash should be on the $#!%$! Desktop, not attached to the Dock.
6. The Dock is one big usability disaster.
7. Using the Trash icon to “Burn” a CD is really dumb.
8. The Address Book is a disaster. You can’t customize the Toolbar. It feels little changed from 10.0. The UI is a complete mess. Do you know how to delete an address book entry? Now let's talk about Mail.app integration ...
9. It’s much too easy to lose the main Window in Mail.app.
10. The sequence of steps required to make a ‘Preview’ window “right sized” is obscure and bizarre.
11. Widgets: Oddly enough, if you’re careful, you can put together a good set, but pressing F12 should hide everything else and the Widget layer should be impermeable - so can’t click through to Desktop or other apps...
12. iTunes breaks a number of UI conventions, but the one of the worst is using the ‘smart size’ Green button to switch to the mini-player instead of resizing the main window. iTunes also disregards the existence of the Dock. I think the iTunes developer team dropped several competence grades in the past few releases.
13. Location information for the Dashboard widgets is set by the address card information for the currently logged in user. This is not documented. Cute and stupid. (The address related widgets don’t work outside the US, and they don’t provide any error messages to that effect either.)
14. Open Mail.app. Then open Address Book. Then create an address. Then in Mail.app click on addresses. Note the newly added address does not appear. Eventually it is recognized, but it takes a surprisingly long time and a few application restarts. Shameful.
15. Create a sticky. With cursor at the end of a line, print it. Notice the last word does not print. Wretched.
16. The "stuck" CD/DVD problem. (If a CD has fingerprints or otherwise cannot be recognized properly, it doesn't mount on the desktop, but it doesn't eject either!)