Another nice feature is that you cannot capture and hold mines. If you want to deprive your enemy of resources, your Hero has to camp on the mine or building. As soon as he moves away, control is passed back to your enemy.
To take permanent control of a mine you have to capture the town that it's linked to (ie. the one nearby).
There's a skill that lets you "plunder" the mine, taking 3 days of resources in a visit. That's nice for a secondary hero to have. You can also destroy resources for a week, depriving everyone of it, which is useful for hit-and-run siege type situations.
I'm still on the fence about the new systems in 6. I half-like the tiers idea, where units are differentiated more by their abilities than by their "level". However it can make some units useless (like Spring Spirits... blech). Also a single Tier 2 unit can wreck house early-game.
Not sure I like how easy it is to heal stuff...
Hero customization is nice, but it makes for some very boring heroes. Random skills could be frustrating but it basically ensured you never got the same hero twice.
I kind of like the zones of control - if you're strong enough it makes it easier to take control of an entire area rather than deal with unitless heroes coming and stealing your mines for a day. I tend to play with one very strong hero and a bunch of secondaries for exploration and resource-grabbing so this helps my style a bit.
I don't like the resource simplification. Before, each town could be defined by a type of resource (inferno = sulfur, tower = gem, haven = crystal, etc). Now with just three types macromanagement is a little meh.
I do like the changes to the battle system, with half-movement/attack morale and hero attacks/spells whenever. It's a nice combo of the HoMM3 and HoMM5 battle systems, but doesn't make morale too powerful like it used to be.
These impressions are all based on playing the demo. I've yet to pick up the full game, and I don't think I will for some time. I got 4 and 5 on release and both of them were buggy messes for the first few months, and the beta wasn't much better only a month ago. I hope the delay let them fix a lot of bugs but I don't count on it.