There's a new lineup of classes, of course, with the only one held over from Diablo II being the Barbarian. There's also the Monk from the original; all classes can be played as either male or female. Your stash, as opposed to being per-character, is now global (transferring items between characters is as simple as putting it in the stash, and then switching over.) So far, I've a Barbarian and a Demon Hunter (roughly equivalent to the Amazon in Diablo II.) Only one class (the Witch Doctor) has Mana; the other classes have class-specific energy reserves which are replenished or charged up in similar ways. At that, there's also no longer Mana Potions.
Health potions have a cooldown rate - after using one, you're locked from using another for some 30 seconds or so. That's not always a problem, as many monsters drop health orbs, which instantly give you and your mercenary health as they are picked up. Also, health orbs and gold need not be clicked - simply run over either, and your character picks it up. (Imagine how useful that would have been in Diablo II, say, in the Forgotten Tower?)
There's a few things to be disliked. One, you must register your CD-Key with you Battle.net account, making it pretty much impossible to give the game away afterwards. Two, there is NO offline mode. You must be constantly connected to the Internet to play at all. Adding friends is a two-way process, like on Facebook, and friends can hop into your games at any time - if you want to go solo, they can interrupt and there's not anything you can do about it, aside from leaving the game. The lack of an offline mode is real simple: It's meant to protect the Real Money Auction House, where in-game items can be bought and sold with *gasp*, you guessed it, real money!
The chat system is carried over from World of Warcraft, which isn't all that bad, aside from the help command for chat showing you some chat commands that only work in World of Warcraft. StarCraft II also uses the same chat system, but not as much of it (Diablo III literally has the same floating chatbox, scroll buttons, behaviours, and font as World of Warcraft. Brand identity, I guess.) You can add friends who play StarCraft II or World of Warcraft, and chat with them in-game from Diablo III.
That pretty much summarizes my thoughts. So here's some screenshots of the game, some of which were intended to be used to report bugs to Blizzard. Enjoy!
Note: Click on a screenshot to see it in larger detail.