If there is one thing that stands out in my mind that Heroes IV beat Heroes III on is the addition of Caravans.
I hated the fact that in H3 if you flagged a dwelling, you would have to send someone each week to actually pick up the creatures, and if you didn't get there each week, the growth disappeared and started back to 1. Sometimes it was impossible to get them as they might be far away or your army was full.
At least in H4 you didn't have to worry about that and could transport everyone to the castle. Plus, the growth didn't automatically disappear after a week.
Any chance we'll be getting that caravan back?