Somebody should fix this overstretched icon in 10.5 - its driving me nuts.
Utilization of more than 3 mouse buttons is quite limited. I want to assign the functions to additional mouse buttons without the help of third party apps. The same could apply to every HID you connect.
Another addition should be the support of h-scroll functionality of current mouse models.
Allow me to to create my own keyboard layout or offer additional layouts. Currently I am using a no name USB keyboard. Of course it has a slightly different layout than the traditional mac keyboard. I am sure new (german) users (who bought the mini for example) are wondering why the are unable to create the @ character with their keyboard. Not everybody knows to use "option + L". I am also unable to create a reverse slash on a traditional german keyboard which is often used while working with the *nix command line/apps and the terminal. (For example to introduce special characters in file names and folders)
Integrate the Audio-Midi configuration program in the sound preference pane. Make it a "advanced" tab or something.
As others already said, the unification of the styles would be neat. I know that every program has something like a own methaphor for achieving certain things, but we still deal with lots of different interfaces/styles - this could be avoided. For example some of the pro apps look like being risen from the OS9 grave. Sometimes Apple even went against it own aqua guidelines. Remember the paragraph about "not using an aqua button in order to create a contex menu". Yeah...
edit:
A plug-in architecture for Front Row would be awesome. Imagine eyetv integration in Frontrow, or support from other third party developers which could integrate their own solution. This would make the mac mini (or every mac which runs Front Row) a awesome HTPC.
Maybe not entirely related to OSX, but to iTunes.
I am just wondering - If the ipod is connected to a mac or the PC... Why are the playback options (iPod content) so limited? You can't do party shuffle with the content on the iPod. You can't share it's content like the iTunes library. You can't even play music/videos stored on your iPod in Frontrow. Why isn't that possible? Honestly, its not the case that I try to "redistribute" the music. Before anybody argues, I see no reason to store my music on two different places (and wasting additional HD space by doing so)