Allow for scaling of text in the UI, especially the Layers panel.
I have to strain to read the layer descriptions. Since I work a lot in After Effects I have to name all my layers. It would be easier to work if I could just glance over instead of working hard to read the little font sizes. I should note that I have a super high definition screen.

Also, make thumbnail size in Layers panel to save between launches: http://illustrator.uservoice.com/forums/333657/suggestions/33328054