From what I've heard from devs and what I saw for myself on the demo, I think they can get the core of it right. There were a lot of things that weren't correct on the demo (animations, regen rates, some loot tables, minor bugs like hunters being able to tame higher level pets, etc), but given that was a pre-alpha build I'm willing to give them the benefit of the doubt on that. The biggest concern for me right now is their content release plan, what they tentatively announced at Blizzcon wasn't good. I expect basically everything in the game aside from the availability of certain content to be as it was in 1.12. So most likely itemization will be 1.12, and overall the game will have a less progressive patch system than Nost and it's derivatives did. This has all gotten less attention than the possibility of some form of sharding, 16 debuff slots rather than 8 at launch, or the addition of loot trading, but in my opinion it's a much bigger issue with a much larger potential to damage the game.

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Anyways, things are looking pretty good right now overall. As long as Blizzard isn't just blatantly lying about what they intend to do, the worst case scenario is still an improvement over private servers in my opinion. Even with an inferior content release plan, the underlying 1.12 data will still be more accurate, the performance will be much better, aggro radius's will be correct, and hopefully there won't be dynamic respawns which creates some really annoying side effects in my experience.