Well finally got into the Beta thanks to Open Beta weekend and here's my two cents on it, for what it's worth.
First things first, the TL;DR version: the game looks and plays great, there are a few thinks I dislike but in the end it's still the best ARPG since Diablo 2.
Now for the long version.
First of all, getting access to the Beta at all was the biggest hurdle. Login failed multiple times with Error 37 and eventually I got to try the beta something like 4 or 5 hours after the Open Beta started. Starting a game timed out 95% of times and it took some real trial and error to get into a game. This is the kind of stuff that I am definitely not expecting to see on retail. I managed to play with all graphics settings on max without a problem at all (which is what I expected) and neglectible lag effects. I did experience some stuttering here and there but it was exactly the same kind of stuff I used to see in D2. All in all for a Beta test the game ran pretty smoothly with no noticeable lag, if you managed to be patient enough to get into a game.
The game's visuals are in line with the current Blizzard visuals and the game looks a lot more like a relative to Starcraft 2 / Warcraft 3 than it does with Diablo 2. The sound design, however, screams classic Diablo all the way. I'm not particularly displeased with the game's visual style but I can see how much the departure from classic gothic Diablo may disappoint some people. It's really a matter of preference in the art style and we can't really say the game doesn't look good because it does.
The gameplay in Beta is, I hope, really not conclusive of what we'll see in later areas / difficulties / whatever. The game holds your hand along the way through the whole experience and the Popup Tips are extremely annoying and definitely the first thing you will do once you get into a game is to go to the options menu and disable them. They are teaching the ropes to people who never played an ARPG before (really, click here to move?) and to people who usually play these kinds of games they are just screen clutter. One thing that really annoyed me in this department was the Journal opening automatically every time I listened to monster Lore; I couldn't figure out how to disable this which is very annoying.
The difficulty seems to be set at "very easy" with monster AI being very basic. They seem to be more interested in standing still waiting for you to drop a chandelier on them than on attacking you, and this is something I hope is an effect of Normal Mode + Early Act One.
The Wizard was my first choice since I've always played casters in every game I played (Sorceress/Necromancer in D2, Druid in WoW, etc.). I can't help but feel a little disappointed in the Wizard; his skills lack some serious punch and I guess we'll have to wait until later in the game to have access to some more impressive skills. He plays pretty much like the Sorceress from D2, including the early "I'm out of mana to cast anything worthy" moments.
The Barbarian is great. I always hated melee characters but the melee hits are brutal in D3. The excellent sound design contributes to this of course, and make playing with the Barbarian a joy. Only thing I didn't like was that most of the time I wasn't paying any attention to Fury at all - I was just hitting stuff with whatever I felt like at the time. I'm hoping this changes later on in the game since it makes Fury pretty much obsolete.
The Monk starts off as pretty uninteresting with his early skills, since I found unruned Fist Of Thunder underwhelming and Lashing Tail Kick pretty much useless since it knockbacks enemies when what I really wanted was to be in melee range with them. Once Tempest Rush was available I started having some fun in conjunction with Blinding Light.
The Witch Doctor is a killing machine. It plays nothing like the Necromancer, surprisingly, since you have to be a lot more active than just summoning minions and waiting for them to kill everything. His skills are totally over the top but amazingly fun. I strongly dislike the constant noise the Corpse Spiders "jar breaking" produces, though. This is probably right now my favourite class in the game but, like the Necromancer, playing with it seems to make the game way too easy.
The Demon Hunter..... geez... I hate it. It's condemned to a couple of specific weapon types and his skills don't feel like they belong in a Diablo game at all. Its only useful skill to kill stuff, Rapid Fire, seems like your firing a machine gun and not a bow/crossbow. I don't like it, I don't like machine guns in Diablo, and I'll avoid the Demon Hunter at all costs. It's a matter of personal taste but to each his own, and I don't want to see guns in disguise in Diablo.
It's still to early to make any comments in regard to itemization, build variety, etc. since we get so little content but the game is indeed looking great and miles away from Torchlight/Titan Quest/whatever. The only problem I have is that I find it extremely uncomfortable to point with the mouse and press a numeric key to perform a skill; I would have preferred to use the mouse only and have the numbers change the active skill, like D2. I'm not sure I'll get used to this, but I hope I do because it ruins a lot of the fun I could have playing this game.
Bring on retail.