I think I've identified the five main reasons that, for me, Diablo 3 feels like a step back from Diablo 2.
Slow-paced gameplay: I've mentioned this already, but the game is just slower than Diablo 2. It's evident in things like the fact you rarely meet more than ten or so enemies at the same time, even with things like the Fallen ( the little demon guys you'd encounter hordes of in Diablo 2 ). There are also lots of enemies you have to wait for. Like skeletons emerging from the ground, or demons being summoned, things like that. You have to wait before you can even hit them, and the wait usually takes about as long as the killing.
Potions are another really important factor here. Because potions are on a 30 second cooldown, and most of your healing comes from globules dropped by monsters on the ground, you quite frequently end up having to run away from strong monsters while waiting for your potion cooldown to finish.
Too much story: I usually complain about games having too little story, but Diablo 3 gets it wrong. Where Diablo and Diablo 2 left most of the story to tomes scattered around and dialogue before and after quests, Diablo 3 injects as many cutscenes and poorly voiced, blandly written conversation as it can between quests that are usually way shorter than in previous titles. The worst part is that someone was paid to write this story, and someone else was paid to record the voices. And they're bad. Hilariously bad.
Exploration and immersion: Of which there is none. I just figured out that the map seems to reset if you leave the game. That is, if you've explored an area and then return later, you'll have to explore it over again. That makes it feel basically like an arcade game to me. Not to mention the world is far less open than in Diablo 2. It has more in common with the narrow environments of Diablo than the open ones of Diablo 2, but without the randomness ( supposedly there is some randomization, but ... I can't seem to find it ) to make up for it. It has more in common with Titan Quest than Diablo in that regard.
Also, this may seem like a minor thing, but it really annoys me that you can't have a map overlay up while exploring. Instead you get a minimap in the corner that you can't adjust in any way ( I would've liked to remove it entirely ) and an overlay that disables movement and darkens the screen. Apparently this was a deliberate move to get people focused on the action, but personally I just find it distracting in that I have to stop to see where I'm going, and it very definitely feels like a less advanced version of what they had in Diablo and Diablo 2.
Achievements and popups for killing x number of creatures in y amount of time is another thing. A product of the COD era of gaming, I guess.
Loot: Why is there crafting in a game that's essentially driven by the desire for loot? Really, I don't get it. And why do items come pre-identified ( except in the case of golden unique items )? I mean, call me simple-minded, but finding an item and identifying it to see if it's any good is a huge part of what makes Diablo exciting. Especially when it's a golden or green item. But Diablo 3 isn't like that. Instead, the best items you get for the most part from crafting them yourself, which is just ... missing the point entirely, really.
Classes and character customization: I got into the limitations of the classes in my previous post, so I won't repeat that. I will say that it also sucks that there's no spending of attributes and skill points, though. The skill system itself works, although again it feels like a more primitive version of Diablo 2 and not a more advanced one. The main thing is that you can't build your character as you want him or her. Leveling up just isn't as much fun when you can't build your character as you want. You don't get to tailor attributes and skills to your own preferences. Every few levels you're given a new skill, and a lot of these are essentially useless compared to others, and then sometimes you get runes and passive abilities which you basically have to pick based on your role. It just lacks flexibility.
It's not all bad, of course. The main good thing about the game is that I get to rant about it, which fills me with much joy. Aside from that, I can't think of a single way in which it's better than Diablo 2. Now, does anyone know where I can get a digital copy of Diablo 2 and Lord of Destruction? That'd be great.
While I wait for Torchlight 2.
I actually thought of firing up Torchlight earlier - while playing Diablo 3. Man, this game is ... It's a decent way of killing time that you'd otherwise spend reading TVTropes or Wikipedia. Or the local newspaper.
Edit: Until now, I avoided talking about the bugs and problems Diablo 3 suffers from. I'll mention something briefly now. As you read it, try to imagine this following part being spoken in a barely restrained manner, through clenched teeth: Blizzard have really created something revolutionary here. Never before have I experienced anything like this. You have not lived until you've experienced lag and server maintenance in a single-player game. It is a sublime experience.