I finished my first run-through of Diablo 3 last week. I liked it alright, not as much as Diablo 1 or 2 but the problem isn't that it's a lesser game, it's more that it's just the same game. It's what it says on the package - Diablo 3, more Diablo - but since Diablo 2 came out like 10+ years ago, games have moved on. I think people, including myself, were expecting more.
I got 30+ hours of gameplay out of it though, so that's fine, and I'll definitely revisit it a bit, maybe play online a little. I started going through the 2nd difficulty level but it's just the same damn thing - the enemies are a little harder but the loot is also better so it balances out, it's kind of a wash. It really is exactly the same as playing through on the first difficulty level though, actually there's even a little bit less content since all your followers and merchants are now unlocked from the beginning.
Boss fights aren't particularly fun either IMO. They're of the old school, just figure out the pattern, maybe die a few times but not too hard.
I went with a Witch Doctor because it seemed the most similar to the Necromancer from Diablo 2, which was mainly what I played in that. Overall I didn't like the Witch Doctor too much - all the loot is tailored for the various classes, so the same item will look different on a Barbarian vs. a Witch Doctor, and on the Witch Doctor it mostly looks like crap.
I started one of each of the other classes and played them all up to level 3 or 4 (15-20 minutes) - of those the Wizard seemed the best suited to me, though the Barbarian was cool too (and they get all the good-looking plate armor later in the game).
Overall expectations were too high, this is partially Blizzard's fault for taking so long to put this game out - I'm pretty sure it could have been released 5 years ago and ran just fine on most systems.
I'm sure I'll play it a bit more. If it was an MMO I'd be pissed at the lack of content, but there's no monthly fee, so really it's just disappointing that I liked it but didn't love it.