I would say it's a collection of things that makes this a disappointment.
- Lack of interesting drops from beginning to end. Diablo is supposed to be a loot fest, this is not. I have no gear on that I found, zero. That would be unheard of in Diablo/Diablo 2.
- Dumbing down of skill system. Why? Because Tweedle Dee can't use more than 1-4 and 2 mouse buttons?
I miss having to remember the 20+ keybinds for all my skill that I'd use. Not every skill was constantly used in D1/D2 but there were times and places for any of them. Ex. On my Hydra/Metor Sorc, I had some gear with + Frozen Orb. I had my array of fire but could whip out Frozen Orb when needed or skip TP and have +Telelport gear item. Etc...
There's nothing wrong with keybinds, it keeps your mind sharp and your skills up. This is like a visit to Sesame Street. I am used to game like Diablo 2, Tribes 2 (where you had to use 80 keys if you wanted to be great), even my WoW setup used most of my keyboard + Clique + Grid in a healer role, keybinds are not scary, we're not that box of rocks you think we are.
- Punishing mobs. D2 had EF,LE,SS,ES Stygian Doll as the OMG I'M GONNA DIE mob but here, it's a little over the top, invulnerable, really? Is that fun? I could kill EF,LE,SS,ES Stygian Doll in D2 with kiting and corner hugging and strategy. Here? VORTEX, INVULNERABLE, + maybe add in MORTAR and or EXTRA FAST or MOLTEN or whatever. Really, is this fun? For whom? I'd rather suffer a steel wire brush enema.
- 4 player games only? Wow, was I disappointed when I saw 4. Also, there's no way to select which games you can join. I'm so sick of joining a 1 player public game where the 1 is afk and or logs as soon as you join. I understand that a lot of games get to fail mode when VORTEX, INVULN, MORTAR, EXTRA FAST appears in an area you need to go, so the entire game disintegrates. It's frustrating.
I'm not quitting yet, I'm going to give it some time, but I am definitely disappointed in a Blizzard release for the first time I can ever remember. Blizzard has always knocked it out of the park when releasing a game. This feels like an infield dribbler.