Honestly, while that's nice, I think the #1 feature that ought to be improved, when it comes to customization of appearance, is: WIDGETS EDITING.

At the present time, with one wrong mouse move, it is possible to send to oblivion entire widgets with their contents, and there is NO restoring them, no "undo" feature.

It is also possible to save, delete, update the contents of widgets with just anything, and if you screwed up something, you're in muddy waters, as there is no going back.

Frankly, I'm getting cold sweat every time I must update widgets on a wordpress blog. Making sure I have a database backup, copy-pasting into save files, triple-checking I don't make wrong drag and drops...
Nothing in wordpress gives me negative vibes like that - nothing.

Here's to hoping that too is improved in the future :)