Further Background customisation would really be a nice little detail. Most importantly, please Larian, give us the possibilty to play at least a group of 5, so we are not restricted to a simple rouge-fighter-mage-healer line up!
As a fan of the old BG games i personally also would really love to have the possibility to use a similar structured and "stone-ish" UI. But, alas, thats just a small a detail, though it would bring back a lot of nostalgia with probably not so much effort.

However all in all it is, as chocamocha said, really fun!