Performance definately needs to improve.
GarageBand is a very similar (albeit much easier to use) program to one I used on my PC - Magix Music Studio 7 (I believe there is a 2004 edition out now with a freeze function).
It ran mostly fine and my PC was a PIII 500MHz with a whopping 128 MB RAM from the end of 1999 (when G4 MHz = Pentium MHz). If that program could run on that, then GarageBand SHOULD NOT DROP A VISUAL FRAME (although I'm glad they gave priority to audio over screen refresh!) running on my 1GHz G4 with 768MB RAM!
I think they really need to optimize the code. That said, the PC did have a 7200RPM HD (I think).
What would I like to see?
1) A freeze function (it's in Music Studio 2004 for PC, RRP £39.99). This is not a pro feature (they'll all have G5s anyway), this is just something to not limit creativity on a less powerful machine (including the whole of their G4 line-up).
2) Panning controls as already mentioned
3) Better performance (as already mentioned)
4) Better MIDI editing controls (like the ability to add notes to a midi sequence already recorded - I can't figure out how to do that currently)
5) Track bouncing (this can also cut processor power. Currently I would have to export to iTunes and re-import to acheive the same effect.)
Overall though, a very good program. My computer (including its 4200RPM drive) has not really had any problems yet but I don't want to find my creativity limited when I start producing more complex songs at Easter when I get to use my keyboard.