Regarding Exposé, I think some people (like me) will never use/like it. My right hand never leaves the mouse except for typing, the left hand has the thumb sitting constantly near the space bar and the cmd key, and it's perfect for me to leave the thumb there and use my middle finger to move quickly from Tab to H or vice versa. I constantly hide and switch between apps, basically moving just one finger, and I'm happy this way. I don't use it, but it doesn't get in my way either; fine for me!
Serious stuff: the Finder and everything else that's already been said about metadata. And FTP too!
GUI consistency, please, I beg you Steve, one GUI to bind them all...

OS X still has huge stability issues, Carbon apps are still able to lock everything up, but worse than this is the mess that surely lies with kernel/usb drivers. I have an iBook G4 and I carry it around all the time, plugging and unplugging loads of USB devices in different places and configurations. Well, after a while things can start to get ugly, leading to Kernel panics. I can reproduce this behavior whenever I want (not that I want it often, to be honest

), and using just kbd, mouse and harman/kardon sound sticks. This bug is here since 10.1, albeit over time it became less evident. But still, that's how I discovered the new panic in panther! Maybe these drivers that load and unload themselves don't clean up memory very well?
Strictly related to stability issues, I'd like a key combination to either kill every GUI process (rude) and revert back to a text only login console, or simply
switch to one leaving the rest intact like on linux: most of the time (excluding the panics) it's just the GUI that's locked, probably only in the frontmost app. It'd be nice to be able to kill it without bringing down other apps in the background or risking damaging your drive's contents!
And for chrissake, I'd love to be able to have
real smart playlists in iTunes, like say, this artist OR that other AND both 4 stars or better. Is it that difficult?
These are the things that really bug me, and the last two don't seem too difficult to implement...

ZoSo
[EDIT: tried to clarify some thoughts]