Being able to move windows and doors in half-tile increments. It's so frustrating when you want a door centered in the front or back of your house, only to have to add one more tile to the width of the house itself because you can't move the door in half-tile increments and the only door that properly fits your vision has a weird coverage footprint for its mesh-- 2-tile footprints on 1-tile doors, 3-tile footprints on 2-tile doors...that gets on my LAST NERVE!!! Also, I wish they'd stop making 2-tile doors that look like they should be 1-tile, and 3-tile doors that look like they should be 2-tile. It's a real pain for a perfectionist like me.
Addendum, Feb. 5: I was so worked up about the windows and doors that I almost forgot this one. Let us FINALLY add colors and textures to the ceilings! It's coming up on 6 years now and we're still stuck with those bland, ugly white ceilings for every build. Even TS2 had paintable ceilings sooner than this...