I've seen it twice. I watched it on the first available screening, which was awesome. Then I watched it again with my Dad a week later (he called me up and asked me to go with him, as he felt that a 47-year-old man couldn't really go to a Superhero movie alone and not look creepy).
Both times it was awesome. I've got to rate it as one of the very best movies I've ever seen, and that's not just the fanboy speaking. The writing was excellent, and all the parts were brilliantly done, especially Ledger. I admit freely I was wondering if he'd pull it off, but I'm sorry I ever doubted it now.
As for the next villians, while signs do seem to point toward The Riddler, I'd like to see a Penguin-Freeze teamup. They both have the "ice" theme, and it seems a logical choice now. Begins was about becoming Batman; Dark Knight was about choosing to remain Batman; the next one should be about choosing to remain Bruce Wayne as well, and Penguin is one of the few villians who'd challenge Bruce instead of Batman.
I'm seeing a rival corporation, like Cobblepot Industries or something, making a power play against Wayne Enterprises. Penguin is what Bruce would be if he used his wealth and influence for personal gain... maybe even more of an evil counterpart to Lucius Fox than Bruce Wayne.
Freeze would start out as just a regular, if infamous, hitman. Have him making money to fund research on his dying wife if you wanna make him tragic. He's hired by Penguin to take out either Bruce or Batman, whicever way it goes; naturally he fails and is badly injured. Penguin, deciding the deal wasn't made properly, changes the wounded man into the ice-themed cyborg thing he's going to become. Freeze and Batman clash, with Penguin trying to catch both outlaws and become a hero in the process while destroying Wayne Enterprises in the crossfire.
...yeah, I don't know. The plot slammed its way into my head about 30 minutes after I watched The Dark Knight. Which says something about either my sanity or the quality of the film. But I am excited.