Regarding Arantir's avatar – it's useless. Throughout the entire mission, it was summoned only twice, and that was just for fun. Compared to the Phoenix, it's roughly equal, but the fact that the hero loses all mana upon its death makes it less relevant.
Okay, I'm being too negative; I just needed to vent a little. Now, let's talk about the positives.
Excellent voice acting. It's a shame that the Russian version will ruin it.
A GENIUS feature is the "K" key for viewing the kingdom; the person who came up with it deserves a monument. I only enter the city with a hero or to visit the market; everything else is done using that key.
Very interesting alternative abilities; it really makes you think about what and how to use them (although invisibility is still useless, except maybe against neutral creatures).
The Orcs turned out great; maybe I'll finally find my favorite Castle in the 5th installment, or maybe I'll still rely on random choices. Although now, it might be worth taking a closer look at the older factions.
I haven't played much of the single-player mode yet, so I can't judge the new Castles. In the campaign, they will only appear in the fifth mission of the first campaign.
Assembled artifacts are great; it immediately reminded me of Diablo II, a successful implementation.
Okay, that's enough for now; I'll play some more in the evening and write again. Or I'll just answer questions if anyone wants to ask.