I'm trying to start a list, but some off the top of my head:
1. Ability to export keywords in something other than alphabetical order. There are many reasons this would be beneficial, but the specific one that comes to mind is Alamy.
2. Ability to see development history in a right-click context menu...or some other means than having to go to the develop module to see it. It's not the develop settings persay, but that is also where export history, print history, etc. is kept.
3. The only place I sometimes see a "slow-down" is when using the spot healing tool. That seems to affect performance far more than anything else in the workflow (for me). I don't know how you make it "faster", but that would be a good area to look for improvement. I have seen others mention peformance issues in this same area.
4. User created fields. Again this would be useful for many reasons, but having another "memo" type field that performs like the caption would be wonderful. Also, being able to create film photography related fields.