On the main screen are a couple new options, Sleep and Elevation. Elevation allows you to flip between above and underground maps and Sleep lets you park a hero on the map, taking him out of the 'Next Hero' cycle. This will finally eliminate the dialogue box that always came up in HOMM2 when you tried to end your turn and had heroes with movement points left, but you didn't want to move them. Also, the information on the main screen has much more detail than before. The towns also have a new function as well, called the town status window.
Armies are much more flexible now.
This is nice when you want to break up a group of flying creatures into two stacks to hamper the ranged attack of an enemy hero's forces. You can also use this when you have a hero with only one stack, but want to give all but one creature in that group to another hero for an important battle. Otherwise you would have to give up an entire stack from the more powerful hero to do this, because all heroes need to have at least one creature with them. This interface makes this process a little easier and up front than before. The combat phase now has a couple of extra options as well.