I've been playing the beta for a month or two. Here's my take:
This is very much a "diablo" game in that they've really captured the gameplay / feel from the previous games. I mean they've nailed it. Click-clicking on loot (which is now personal to you) and destroying tons of monsters is still super satisfying. The environments and spell effects are beautiful. The "destructible" environment feature they've touted is sorta meh to me, it rarely gets used to destroy bad guys. Stellar art direction -- the graphics are beautiful but not in a Crysis graphics-card-crippling way -- typical Blizzard game in this regard and they hit it out of the park as usual.
They've added achievements as is in fashion in games these days, the ones in the beta are pretty cool but don't seem to confer any benefits on your characters -- not sure what they have planned for the release version.
There are some fun "events" that can pop up when you're going through areas -- I like these a lot (jar of souls for one) but many playing the beta seem to want to skip over these and just run as fast as possible to the next boss.
Crafting seems useful as you're leveling, but they recently removed the ability to disenchant while not in town, and normal "white" items are no longer used to disenchant and then re-craft from the resulting materials (instead you end up selling white / grey items in town for cash). Not sure how I feel about this change, but it's not a huge issue. What's cool is that you don't craft yourself, you have a blacksmith that follows you around the acts and levels up as you spend gold on them. And at least in the beta, the same blacksmith is used across all characters (e.g. no need to level multiple blacksmiths)
Grouping is easy using their matchmaking system, but there seems to be no global chat as there were on the D2 battle.net servers. This is probably a good thing mind you
The net code seems a little buggy -- a few releases ago, I got very little lag but now it stutters a bunch, especially when starting a new game. I know they just sent out a bunch more invites, probably to perform load testing, so this is to be expected.
The auction house UI is terrible for browsing items. I really hope they fix this prior to release. Otherwise the AH is a pretty cool feature. I'm not sure how the real money vs. gold AH factor will play out, in terms of what you'll be able to find on each. It seems like the sheer number of loot permutations will keep this game interesting, and make playing the AH worthwhile.
In terms of classes, each plays similarly to a class from D2:
D2 Sorc = D3 Wizard
Amazon = Demon Hunter
Paladin = Monk (each can heal, but monk has more ninja-like attack skills)
Necro = Witch Doctor (similar in that both summon creatures)
Barb = Barb (almost identical)
I'm enjoying playing mostly the wizard and demon hunter. Both are super fun, even up to the level 13 cap, and you can see the potential when the more bad ### skills get unlocked later on. The wizard is like a typical RPG glass cannon, but what's cool is that their "mana" (or arcane power as it's called I believe) regenerates really quickly so you're never far away from nuking something. The demon hunter's mana ("hatred") regenerates a lot more slowly, but they also have this other mana thing (discipline) that activates other types of abilities that don't require shooting (like evasive maneuvers and ####). That, and you can dual-wield one-hand crossbow guns, which is awesome.
The main problem with the beta (well, not a problem really, as I'm certain this was intended) is that it only contains maybe 10% of the game (on normal difficulty, so it's VERY easy), maybe 25% of the skills for each class, and no runestones which will be used to customize skills. Customizing through runestones and playing on the harder difficulties are two big factors that are going to give this game legs. The beta gives you a taste of what is likely to come, which is potentially awesome, but you can't be 100% sure you won't burn out on it long term.
In short, I wouldn't expect anything earth-shattering in terms of novel or unique gameplay, but if you're looking for a modern version of D2, with all the improvements that modernity brings, you should be very pleased. I rate this a BUY