I'm going to keep this short because, well...it's 4am. I just beat the Diablo 3 Beta with the Wizard class. My overall impression: this is a HUGE improvement over Diablo 2.
Pretty much everything except what I talk about below. Navigation/movement is pretty much the same as Diablo 2. I did notice that there wasn't a good way to walk around with the map displayed, which is something I will miss, but it doesn't kill the game or anything. The effects for abilities were really solid, and the physics are amazing.
I ran into a couple of spots where the gameplay seemed a little unbalanced in the enemies' favor. For instance, the areas where Crypt Guards are encountered - they can spawn a shit ton of enemies, and if you don't know to kill the Crypt Guards first and they get to spawn a lot of skeletons, then the game turns into a kamakaze sort of thing where you run headlong into a gigantic army of skeletal warriors, take out as many as possible, die in the process, and repeat until they're all dead.
The idea of having to be connected to the internet to play this was not bad. The idea of experiencing lag symptoms when you're playing a single player game is. There were a great number of times where the game would freeze up, presumably because I don't have the greatest internet connection right now, and I'd get owned by a really weak enemy. There were also a lot of times where I'd be running in one direction and I'd periodically get moved back to where I was a few seconds ago. I don't care if I have to be connected to the internet, but please please please please don't make movement/combat controls dependent on server communication for a single player campaign: that's just silly.