Graphics - they're great, just not for diablo. They look more suited for WoW than diablo if I'm perfectly honest. The colors are too bright, and ruin the mood rather than set it. It's supposed to be a dark game, so the graphics are brightly glowing...makes sense.
No sense of community - In d2 you were forced to interact with people and trade. There was a real sense of community you could see and feel. In diablo 3 it seems very isolating, if you're not playing with friends the closest thing to interaction with others is a little public chat window at the menu screen or joining a random game where you just go through the campaign. Where is trading, where is dueling? How am I supposed to show off all my epic loot?
Unbalanced - simply put, classes aren't balanced, especially in inferno. Any ranged class has a huge advantage. Barbarians especially get the shaft in inferno, their solo survivability is horrible. Even with the best gear off the AH, including legendaries I'm 2 shot in inferno. When I could be blazing through it on a wizard without getting hit at all. Which is why people have already solod diablo now as ranged...mark my words, you will not see 1 barb solo it for a very long time.
Elite packs - there should be limits on magic property combinations, getting jailed on top of an arcane orb or pulled back into a freezing bomb etc. things like that are completely unavoidable for the most part. Especially for hardcore characters, it should be changed.
Bosses - Easier than elite packs, what's the fun in that? They feel a lot less epic and anticlimactic now. Why don't you just rename elites to "bosses", it all seems backwards.
Monster scaling - Instead of making monsters smarter in inferno or setting things up so there is actual skill involved they just increased the stats of everything. Really lazy, and we all feel it in the game play. Classes aren't individually adjusted for the stat changes either, which really makes you want to face palm.
Auction house - really should never have been put in. It has turned diablo from adventure into mindless gold grinding. The whole gambling like aspect which made d2 addicting is out the window, I don't even care what drops really. All I know is I'll be buying and selling everything on the AH. Not only was it a bad idea to implement an AH into d3, they didn't even do a great job with it. Limiting items to 10 hasn't stopped people from manipulating the AH prices, it just pissed everyone off. I've bought every stash space and it's completely full of items I wish I could sell but I'm afraid to. It all feels like a chore.
Items - Blues aren't just as good as legendaries and set items, they're better...and not just a little bit better. Where is the sense of direction here, why did you even color coordinate anything. They're better than crafted items as well and cost way less on the AH...which instantly makes crafting next to useless. It all makes absolutely no sense, you guys really messed up on the items big time.
Exploits - Town portal and leave the game then re-enter and elite packs/chests are back again. Most people I know farming gold just do this over and over again instead of actually playing the game. Co-op rezzing on bosses, and other exploits being hotfixed as well, they're so obvious and in your face that the game really seems unfinished.
Crafting - what's the point when everything is on the AH. Especially when the stuff you can make with the blacksmith is so underwhelming compared to AH items. It cost 50k + materials just to make a chest armor that's 2x worse than one at the same price on the AH.
Overall I came into diablo 3 very excited and hoping for the best and it was fun for the first 30-40 hours. But for all the reasons above, I've lost interest and I believe many others will as well. I hate to say it, but I believe there was just so much WoW influence that it completely obliterated the game. Crafting, legendaries, 60 level cap, auction house....it's not a coincidence. Good bye sweet Diablo series, forever rest your soul.