- Performance tuning and stability. This is a given, really, as every game goes through this process prior to going gold. I'm pretty happy already on my fairly modest hardware (i2500K, GTX 770, 8GB RAM), but there's always room for improvement.
- Army behavior. AI-controlled armies are the second major selling point in this game, after scale thanks to advancements of the engine. There are various posts already showing off wonky army behavior that should be addressed. Currently there is too much of units shuffling around, hanging back in fights, going backwards to get into formation, not to mention going up cliffs. Furthermore we should have better controls for grouping and splitting armies.
- Unit and group responsiveness. There are a number of threads from new and old players alike commenting how it frequently takes multiple clicks or clicking in the right spot to get units to execute an order. I consider responsiveness a key factor for a release-quality game.
I have, of course, tons of other improvements I'd like to see, but I consider the above core to the game and necessary for a release-quality product. The rest can be addressed after release.