You have to trust Blizzard with their games. They often aren't revolutionary, and you can almost never tell from the footage how fun they are, but they ALWAYS get the gameplay right. That's not an exaggeration--they are the most consistent game development company. I'm sure that could even be proven by a quick browsing of metacritic, but regardless, almost all of their franchises are listed in nearly every 'Best Games of all Time" list and are played for years and years.
The footage did look amazing, though. After playing Titan Quest I learned how much more fun a Hack 'n Slash can be with visceral combat, and D3 appears to take it even further, with the Barbarian jumping around, dispatching multiple enemies with a single swing and plowing through hordes using a variety of attacks that all work in different ways. Compared games like the original Diablo or Dungeon Siege, that's a whole different experience. It just feels less stats based and more satisfying. Although I understand that's not how everyone likes it...
tl;dr: It's a Hack 'n Slash, and the hacking and slashing looks amazing.

I just hope it will focus more on the dungeon crawling and offer random quests like the first Diablo did. D2 got stale because it was all about doing runs for XP and loot over and over again, trying to avoid dungeons as much as possible. The first Diablo on the other hand was about giving you a sortof short dungeon crawl but every adventure would be different, from the quests you'd get to the enemies you'd encounter, and it was impossible to see the whole content in a single play.
There is a reason why I still play it today.

Despite being randomly generated, Diablo 1 did a great job of building a deep, dark, atmospheric dungeon to explore. Diablo 2 did not.
Most hack-and-slash games at least put up a pretense of not being about just left-clicking for loot, but Diablo 2 pretty much boiled the genre down to its most insoluble elements, then built a whole game out of nothing but that. Loot was important and part of the fun in Diablo 1, but in Diablo 2, loot-whoring was absolutely the only point. Sockets, sets, gambling (Wirt to the tenth power), rune words, transmutations, imbues, elite items - all the gloom and doom of the original was replaced by new and different ways to twink yourself. It was so over-the-top that it almost felt like a parody of itself.
Even the drop mechanics were modified - magic items dropped a lot more frequently than in D1, but unlike D1, special properties had no maximum level at which they would stop spawning - so you could easily find a rare pike in Act V nightmare with +2 hp, +1% light radius and +1 armor class. This served to make the game even more loot-whorish by making it so tedious to sift through the metric crazyton of magical junk in search of the worthwhile drops.
But, that's just my opinion.

Personally, I classify both the so-far released Diablo games as "Arcade" - together with Tetris, Space Invaders, Breakout, et al. The gameplay is equally simplistic and the main mechanic seems about the same - shoot everything, and finish this level so you can enter the next where the enemies are slightly tougher, faster, or both (compare with Tetris where the blocks fall faster as the game progresses, or SI where the aliens get quicker), and then there's a boss every now and then. It doesn't even have a hard-to-learn 20-button control scheme to differentiate from the aforementioned arcade games, just left- and right click the enemies one after another (actually, the video with the "Mommy mode" of some other game that someone posted here somewhere comes to mind).
The loot and so-called "quests" are just Blizzards try to badly disguise it as an "Action RPG".
Note that I don't despise the Diablo games, It's the simpleness that makes them fun - just like Bejeweled, or the bundled-with-windows Minesweeper or Solitaire.

There isn't really a problem with the action RPG formula. Yes, it's arcadish, but is it really a bad thing? Basically it is a more action oriented adaptation of the old roguelikes.
Loot and quests help to keep interest in the game but as Mentalespy said, it's about how it's handled. Diablo didn't throw that much loot your way and so gave it some kind of importance, while the sequel just threw so much that it basically became a chore to go through it, even if you were still basically one kill away of getting a better item than what you were currently using.
As for the quests, which are really quests even if it seems you do not really agree with it — after all, a quest is a search or pursuit made in order to find or obtain something — most of them are quite simple but using only a few randomly picked each time, as it was done in the first game, really helped to give a different adventure since you never knew what was going to happen. Right from the start of the game you may have to solve the poisoned water quest or kill a behemoth like The Butcher, but you'll never be able to encounter them both in the same game.
It's not about disguising it into anything, it's simply about taking RPG elements and give them a more fast-paced action gameplay, in contrast with the mostly tactical rendition of others dungeon crawlers. One of the greatest strenghts of D2 over the original was the diversity of classes, which played differently and could be developped in different ways, and that is one of the key features of what makes an RPG — sure, one of the problems of the game is that it tends to favour mostly one or maybe two builds per character but that's mostly a problem with the execution.

I loved the atmosphere in the original Diablo, and I liked some of the monsters better too. It really was a wonderful trip to a spooky, forlorn place. Wandering through yellow deserts and bright jungles in D2 didn't compare at all. The original felt much more like an adventure.

D3 is going to expand on the original formula of dungeon delving. Using a static world with random quests/dungeons and monster placement. Its basically Titans Quest world with Diablo dungeons.
Titans quest got old after the 3rd play through, this is my only fear. Since D3 is totally static outside of Dungeons how can there be the replay value of D2 at all? I've literally not gone more than a year without spending a few months cracking out on D2 since it was released almost 10 years ago.
The static landscape terrifies me. Its what killed Titans Quest IMO :(

Yes, I have a question: I have never finished Diablo 2. Is this a problem?
Thanks in advance for harsh realities/truths.

For me what killed Titan Quest was the (mostly)passive skill system and the fact most of the world felt like one long corridor. I can remember the beach sections were especially bad for this, not much room to explore just keep on running in one direction. The fact the game had no PvP until immortal throne probably didn't help either.
I still kind of trust Blizzard to deliever a great experiance with D3, if they match my experiance with D2 which I played way too much is another matter. I'm of the stance of wait and see.

Diablo 3 will be amazing just like Diablo 1 and Diablo 2 were. Their games always offer massive replayability and are fun as can be. Blizzard simply does not make bad games, and this game will be amazing

I love the smell of fanboism in the morning!

D2 1.13 patch has been due for months now... on a bright side the patch server is up and running... i have no idea what content its adding but I'm excited for a new patch to D2.. what ever it brings...
Makes me long for Hellgate actually.. I love that game like a fat kind loves cupcakes and ice-cream.... and Flagship had to go and muck it up :(

Ha! I thought that kinda sounds like Slavic behaviour. What do you know, it is :D

My personal name for this is "Slavic Piracy Curse".
I consider Ukraine + Russia being current piracy bastions so yeah, it is probably Slavic behaviour.

I'm pretty tired of the entire action-hack genre, to be honest, and have been for quite a while (depending on what sort of games you want to include in that definition). Really, the only ones I've played that I felt got it right were Diablo 1 and Sacred 1. For me, it's one of those genres where, if the game is not exceptionally well-done, it's just kind of boring.