Well, I’ve played through the beta with four of the five character classes, so I’ve got a solid enough basis to gather my thoughts.
First off, the required internet connection is pretty annoying. I haven’t decided if it’s a dealbreaker or not, though; the game itself is good enough that I’m leaning towards not. Of course, this means that even if you’re playing in single player mode, if Blizzard’s servers are down (say, for maintenance) you won’t be able to play.
The level cap for the beta is set at 13, and everyone is restricted to Act I; the final boss is the Skeleton King (the final boss of the beta, that is; whether the Skeleton King is the final boss of Act I, I have no idea). This is high enough that you can unlock 2 (0f 4) extra skill slots, and 1 (of 3) passive skill slot.
Unlike Diablo II, there is no skill tree; each class has 3 categories of skills (for the Witch Doctor, it’s Physical Realm, Spirit Realm, and Command); each skill in each category is learned at a certain level. However, the player may only have 6 active skills at a time; these six can be chosen from any of the three categories, and can be of any required level. The player can change skills out at any time (though this does trigger a 30 second cooldown on the skill chosen, so that you don’t try swapping abilities in and out during battle). The passive skills are similar, only their effect is constant (instead of triggered, like attack spells are). The skills default to the right and left mouse button, and 1-4 on the keyboard (5 is set as the health potion button).
The inventory system has also changed since Diablo II; instead of every item having its own grid size, everything fits in either 1 slot or 2 vertical slots; this makes inventory management far less of a headache. Like D2, D3 has a stash; it works like the bank in World of Warcraft. You start with a couple rows, and can upgrade it 2 rows at a time until you have a full page; once you’ve got the first page, it looks like you can continue buying additional pages (at least 2 more right now). Unlike Diablo II, the stash is shared amongst all characters on the same account; if you get a great fist weapon on your Demon Hunter, you can just stick it in the stash, switch to your Monk, and get it out of the stash (this is also how the shared stash in Torchlight worked). This is a great improvement; I know in D2 some people had multiple copies of the game and would dual box on Battle.net to make swapping items between characters possible. Aside from being needlessly expensive, this could also make you lose your items (if the server crashed during character swaps, for example).
The graphics are greatly upgraded; that’s not surprising, since Diablo II was sprite-based and came out in 2000. Diablo III is more modern, and in addition to the overhead isometric view, you can also zoom in for a much closer view of your character and its immediate surroundings. This is handy for taking screenshots, but not so useful the rest of the time; it really hampers your field of view.
The character classes are almost all new; the only returning class is Diablo II’s Barbarian. The Monk plays like a hybrid Assassin/Paladin; it’s focused on melee combat, but has mantras that function much like D2’s Paladin auras (they provide passive bonuses to the Monk and other party members while they are active, and only one can be active at a time). The Demon Hunter is a ranged physical damage class that mainly uses crossbows; its abilities are geared towards staying mobile and out of the range of melee. The Wizard plays a lot like the Sorceress; it gets old standby spells like frost nova and ice armor, and one of its passives slows any creature hit by any spell. The Witch Doctor is reminiscent of D2’s Necromancer, but with less of a focus on summoning a gigantic army of the undead.
I enjoyed the beta; I’ve only played the single player campaign so far, but I’ll post more once I’ve tried out the co-op multiplayer (and once I’ve taken more/better screenshots).