Not the guy you are replying to, but my experience having played plenty of D2 and D3, and now a couple of days into D4;
Its very highly polished as I would expect from Blizzard. Excellent production values.
The setting and vibe is much more back to the classic Diablo grim, gothic feel. If you ever felt D3 was a little bit too colorful and happy at times.
From a gameplay perspective I ever liked D3's skill system. D4 seems to give maybe a little bit more customisation, but nothing compared with the old D2 skill trees which I actually quite liked.
The big issue for me is going to be that the dungeons etc arent randomly generated, and even having only finished the first act I am seeing reused assets (every cellar looks the same etc). Im concerned by the replayability, with that in mind. I can still remember a few of the dungeons from Early Access.
Second potential issue is how they are dealing with legendary/unique items. It seems now that rather than uniques, you just have legendary 'aspects' (basically like suffixes or prefixes) that can be rolled onto an item. So you can find a helm with a unique/legendary perk on it - which will mean the helm is legendary - but it will otherwise just be randomised like another other helm. You can extract the legendary perk and stick it onto another helm. I dont know if this changes later on though, as I saw something about new/different items in the higher difficulty levels. Also dont know if this means they are doing away with sets. Cant say I prefer this system.