Ok, now that you've watched, there are two big things that people are talking about. First off, we appear to see Rachel Dawes (Maggie Gyllenhaal replacing Katie Holmes) being pushed out a window to her death by The Joker. Even if this is her death scene, it'll be the least surprising surprise since Anakin Skywalker turned to the dark side. Everybody and their cousin presumed that Rachel Dawes was being kept around in the sequel for the sole purpose of being murdered by The Joker at about the halfway point. This would be the ideal way to establish The Joker as a hardcore antagonist and to make the inevitable showdown between Batman and Joker as personal as possible. Say what you will about having Joker kill Wayne's parents in Burton's version, but it gave the final confrontation an immediate dose of twenty years of built-up pathos. Still, I'm kinda shocked that they are even hinting at Dawes' fate in the trailer.
Second of all, we see Harvey Dent (Aaron Eckhart) being held face down on the floor while something (gasoline or acid) spills toward him. Later, we see a side profile Dent in a car, gun in hand, with just a hint of something very wrong with the other half of his face. This has caused much discussion on various sites and boards regarding just how Dent gets his face disfigured. Let me be frank on this. It doesn't matter one iota whether it's acid (like the comics), fire (like the 90s cartoon), or an allergic reaction to peanuts. These are the kind of nitty-gritty debates that make film geeks and comic geeks look stupid. If the transformation makes sense in relation to plot and character, then the minor details don't matter.
Nolan, Burton, Schumacher, and the Timm/Dini/Burnett team changed all kinds of little details in their respective Batman films and cartoons, and that's not why they succeeded or failed. Batman: The Animated Series radically altered the origin of Two-Face, both in the manner of his creation and who and what caused it, and that still stands as the best version of the character yet done. Batman Forever had an origin scene literally identical to the comics; right down to the ridiculous scene of Batman sitting in the back of the courtroom and leaping to Dent's defense. Yet that didn't excuse Tommy Lee Jones' over-the-top Looney Tunes miscalculation of a performance. It doesn't matter if Peter Parker has organic web shooters or mechanical ones. It doesn't matter if Wolverine is too tall. It doesn't matter if Kingpin is black. It's all about letting the movie do what it needs to do to work on its own as a movie. Period, end of story.