User Interface improvements (anything from new messaging system, tactical battle-orders screens to hotkeying, bloodhunting/slave micromanagement issues and order-tracking)
My other was changing provinces to be "areas" instead, and have them in the shape of a hexagon. Trying to hold a front against an opposing AI with having to worry about a squiggily border line of a couple of provinces is annoying, and not un a fun strategic sense, but in an ANNOYING sense.