My 2cents on all this...
I waited the better part of a week before even trying to go to the theater... went on Wednesday night after work. Every show over the weekend was sold-out in advance at my local theater. I've NEVER heard of that before. But how much of that was the "Ledger hype" and how much is going to fade away after it ceases to be seen as an "event" and is seen as "just a movie?" I'd guess that the peak opening-weekend profits are going to drop and while successful, it's not going to see a lot of "repeat viewings" like the really BIG movies do.
The film was terrific, that said. There area few things that bugged me, but overall, I liked it.
1) I'm not sold on the new batsuit. Yes, it looks functional, but it no longer has that "monstrous" quality... the fear-inducing element. It's just a really cool SWAT suit now. As for the "head turn" bit, well... if the neck part of the cowl/cape portion of the costume were simply (as shown in the comics) made of the cape fabric rather than of thick "muscle-ish" rubber, the head would turn easily enough anyway.
I still think that the best-looking "batsuit" is the one in the web-based "Batman: Dead End." (Of course, you need a VERY specific sort of actor to wear that suit.)
2) I thought it was amusing how they took two things DIRECTLY from the earlier Batman movie series... the "transformer from car to motorcycle" thing and the "joker falling to his death" thing and did a riff on them. I really liked the Joker bit (I groaned when I thought he was going to die that way... but smiled when they turned it around). I didn't like the "motorcycle" bit, however... as an engineer, this just seemed stupid. Remember that the vehicle was built for the military as a bridging vehicle, not built by Optimus Prime.
3) I think Ledger's performance was great. I still don't think he LOOKED the part, but he ACTED it perfectly.
That's the real reason I think it CAN be recast, though... Ledger's performance didn't create a new version of the Joker. Rather, it was the first time that the character as he's existed in the comics for pretty much the entirety of his existence was portrayed accurately (ie, as a "force of chaos" rather than a "funny criminal") was seen on-screen.
What Ledger's performance did wasn't "define" the Joker, but rather it DISCREDITED the bad versions that other actors (including Nicholson, IMHO) have given us in the past. Any future Joker won't be Ledger... but it will be the anarchist force of chaotic evil... the demon in clown makeup... rather than the "funny, entertaining guy who steals stuff and sometimes hurts people."
It's a high bar... but it's not LEDGER'S bar. Ledger is just the first guy to make it past. The next guy may do even better.
4. The penthouse/garage idea for the "branch batcave" downtown is straight from the comics. It was well-done. My only real issue was with Wayne sitting in the suit in front of the windows. Having lived in downtown Chicago myself, I know just how "imprivate" that sort of life could be. Odds are, especially since people would know that was Wayne's apartment, you'd have photographers focused on it at all times. And the next day, he'd be on the front page of the National Enquirer.
5. Wayne Tower wasn't the same building as the first movie... not remotely. Now, maybe this was just some office space purchased to occupy while they restored the outside of the REAL "Wayne Tower" (with the rail system running into it and the waterworks underneath) which was damaged pretty severely at the end of Batman Begins. Still, I'd have liked SOME reference to the first movie's tower, even if just in a background shot, under reconstruction (similar to how they addressed Wayne Manor).
6) I agree that the recasting of Rachael wasn't a drawback at all. I'm not a fan of Katie Holmes, but I'm not a fan of Maggie either. I don't think either one is particular spectacular looking... both are, really (when not made up) fairly ordinary-looking (that's not an insult... most folks are ordinary-looking after all!) The actresses look and sound just enough alike that I could accept the recasting without any "suspension of disbelief" issues. Of course, I also strongly suspected that what would happen to her was what was going to happen, so it didn't seem like all that big of a deal anyway.
7) Eckhardt as Dent... other than his hair being a bit too "emo" for my taste, and the loss of his eyelid (which looked cool but wouldn't allow his eye to remain, much less to remain functional!) I liked this. I'm convinced that we WILL see this character again, and probably played by the same actor. The makeup will be a bit different (from "raw fresh wound" to "massive scarring") but should remain REAL rather than like the prior "batman movie" series gave us with Tommy Lee Jones (aggggghhhh....)
The one thing that I think that they missed, at least a bit, was Dent's psychosis was too emotional. Of course, that's OK because it was a NEW psychosis... eventually, the guy becomes pretty much totally emotionless. Instead of acting on his feelings, the coin makes EVERY choice for him. Good or evil, nice or nasty, generous or greedy... all from the coin. That's the other reason I don't want him "gone" after this movie... there's a lot more development to be had from him. It'd be a waste to lose that now.
Basically, this was a terrific movie. The only things that bugged me were the "setup for the sequel" ending and the fan attitude about Ledger overshadowing the CHARACTER of the Joker (which, I'm sure, will fade soon enough... the character was there before Ledger - and I don't mean the Nicholson, or Romero for that matter, versions... I mean the COMIC version!), the "too techy" batsuit, and the goofy "transformers" motorcycle conversion.
A very few quibbles. It is a terrific movie overall.