OS X:
This is more a problem with individual apps, but it’s consistent enough across the system that I’ll include it: “maximize window.” When you click on that option, the software will resize the window to what it thinks is the right size. It’s usually wrong. If I want to have my Safari window fill the screen, I have to manually drag it.
As mentioned, the open/save dialog isn’t so great.
Pressing return or enter when a file is selected in Finder doesn’t open the file but instead renames it. I actually appreciate an easy shortcut to rename files, but the OS needs an easy way to open documents from the keyboard.
Inconsistent keyboard shortcuts: in the save/open dialog, command-d goes to the desktop. In Finder, the same command duplicates the selected file, you need command-shift-d for the desktop. Of course, I always get it wrong when I have that 2GB movie file selected.
Leopard broke the updater. Nowadays, you a window pops up at the worst possible time to ask you if you want to update your system, which requires an immediate reboot. Previously, you could first download, and reboot later at your leisure. There’s probably a good reason they’re doing it this way now, but from a user’s perspective, it sucks.
Incredibly geeky and probably annoys about 5 people in the world: rather idiosyncratic interpretation of the IIDC specs, which causes Quicktime to not play well with some otherwise standard imaging equipment.
Windows XP
There are too many, and thankfully I don’t use it much anymore, but here’s what sticks out:
In the start menu, the program list frequently obscures the shutdown button, if you’re not careful. The new items added notification also does the same. This is tremendously bad UI design.
As mentioned, the bottom right corner bubbles are awful. No, I don’t care that I have icons on my desktop I’m not using. Stop bothering me.
Stupid and inconsistent keyboard shortcuts. Alt-F4 quits an app. Splendid ergonomic design. However, Ctrl-W closes a window. Please make up your mind, which scheme are you following?
Antediluvian search functionality. It takes Google 2 milliseconds to search the entire web, but Windows takes half and hour to look through my hard drive.
Windows update runs in a background process, sometimes causing your system to run at a tortoise pace without you knowing why (or seemingly being able to do anything about it). This always happens when you’re demonstrating performance improvements to CPU-intensive multimedia applications to a group of 500 people.
The awesomely bad Japanese keyboard layout. Macs have two extra keys, on either side of the keyboard. One switches to roman alphabet, the other to Japanese. You’d think this is all you would need, but no, the Windows keyboard has no less than 5 keys related to language toggling. The one you use the most is of course, in the hardest to reach place.
The registry, which invariably ends up filled with junk that somehow slows your system down.
The too-frequently-used Device Manager is hidden in sub-menus. Which leads to the next item…
The absolute worst, is the utterly incompetent driver model. For several devices, I’m forced to go through the new hardware found wizard every single time I plug it in. Seriously, whatever happened to plug and play?