Wow I definitely didn't see the crowd of negative criticism coming. Everyone I know who's seen it (which is twenty some-odd people) loved it. My friend and I walked out of the theater and basically called it various forms of awesome for about 15 minutes straight. It's easily in my top 10. Alongside some of the other movies listed in this thread (Forrest Gump and Good Will Hunting specifically)
Batman Begins was good, TDK was fucking awesome. I just got sucked into it more than I did in Batman Begins. It was entertaining when it needed to be, serious when it needed to be, and downright fucked up when it needed to be (mostly due to Ledger). Plus you can analyze it to no end if you're into that kind of thing (granted you could do that with BB too).
Everyone who said it was cheesy, uh... it wasn't. SOAP Operas are cheesy. Batman Begins had a few cheesy moments (thank you Katie Holmes). TDK had next to zero (if any). A lot of what you may have interpreted as "cheese" are actually valid literary themes. Even then they were hardly presented in a "cheesy" manner.
As for corny... how so? The Bank robbery scene was designed to be entertaining obviously. I laughed and cheered my ass off. Same story with the Joker blowing up the hospital (although that was no cheering and more of a WTF laugh). The Batman/Joker/Dent chase was anything but corny, neither was the climactic battle.
The plot was complex as hell, the characters were complex as hell, the acting was excellent on all sides (with Ledger singled out obviously. The guy deserves an Oscar dead or not). This movie was well deserving of the hype it got. I wouldn't call it the "best movie of all time", but it's up there.
For people who say it was too long, well, IMO you can never make a movie too long unless it absolutely sucks, but that's me. If they made a 12 hour version of each LOTR movie to include more content from the books I'd gladly wait for a day off and dive in, maybe pause it to take a piss a few times. Just keep the pizza/popcorn coming and I'm good.
And DomS, sounds like you were too confused by the plot to make an accurate judgment. From the sound of your posts you missed entire scenes.