I'm surprised nobody has mentioned this one before (or maybe I just didn't see it) but one thing that I think OS X could do well to adopt is the way Windows handles hot keys for menus. I'm talking about Alt+F for File, Alt+E for Edit, etc. In case you're not familiar, in windows, all the application menus (and certain options) have a key associated with them that allows them to be activated using the keyboard.
For example, let's say that I wanted to select all the text in a document. On Windows, I can hit Alt+E, then while still holding Alt, press A (the key for select all). Or I can just hit Ctrl+A, but that's not the point, I'm just using an example.
Granted, I'm more of a keyboard user, and as such, am very used to hotkeys (and FWIW, I like the Macs Command over Ctrl implementation better, after having years of experience with each, but that's neither here nor there) but sometimes I forget what a shortcut is for some hardly used option, or it varies from program to programs (IE: TextWrangler, by default, maps "Save to FTP" to Cmd-Shift-S, instead of Save As... which is what I'm accustomed to). On a Mac, I'd first have to enable Keyboard Access (why isn't this on by default? More confusing? Maybe, but it's hardly something I see tons of so-called "average users" doing by accident, but again, I digress) then hold Ctrl, then hit F2. (on my PowerBook, I also need to hold Fn). And even that only selects the first menu, I need to arrow through the menu options and hierarchy. Lord help me if the option I need is buried 27 items down in 3 submenus (hyperbole here folks), but still.
Just my two cents.
What OS X Could Learn From Windows