Absence of lan play, offline play. A very essential part of the game, Imho.
LAN is a mark of the past, that's a fact. So is playing D3 single player. Even D2 was so boring in single player I barely managed to finish it.
Not the same gritty atmosphere as in D2. What i have seen of it, D3 is More cartoon-like.
I think you did not play the beta then. I agree, the style is a bit different (it was made by creators of WoW) but they've managed to keep that nice atmosphere from D2. Unfortunately the D1-feel is long dead, that was something perfect.
The way the potion mechanics have changed.
Can't argue here, but actually I like those cooldowns. Previously you could stay in one place and just spam potions. Now you can't do that and you have to plan your battles.
And there is something about the 'feel'. It looks more sluggish than its predecessor.
Actually the game is very dynamic and you can feel the power of every single character. I prefer to play as a caster, but I really enjoyed playing as barbarian and monk as much as a sorceress.
To me D3 is an interesting experiment and it is in some ways a logical progression of how the game industry is now.
After this first heavy sentence I was going to reply tl;dr; but your post was actually quite interesting :)
You've used a bad wording though. Blizzard don't "experiment", they "do". Fans will still buy it. They have earned a huge reputation, they are one of the two developers (the other one is Rockstar) I'm ready to pre-order anything they release.
From my experience I can tell one thing: great majority of people who complain how bad the game is, how much they don't like it, how they've changed everything which was good in the two previous parts never actually played it. It's just complaining for the sake of it.