It was obvious that Heath Ledger as the Joker was going to be amazing just after watching all TDK trailers.
Few would suggest that Ledger's performance as the Joker was anything less than extraordinary. Could you have said the same about, say, Maggie Gyllenhaal's performance from the trailers alone?
Honestly, I don't get why there's any cause for concern. Aside from the single stumble of Katie Holmes, which was subsequently corrected, has Nolan cast any major player in this entire trilogy who's been less than brilliant? Hasn't he earned a degree of trust in his judgment and his ability to elicit fine performances from his cast?
Besides, Ms. Hathaway's been nominated for quite a few awards in her career
-- far more than Ms. Holmes, competitive with Ms. Gyllenhaal. She clearly wasn't cast for looks alone, although I daresay she's the best-looking Catwoman since Julie Newmar.
And hey, she has the same name as William Shakespeare's wife. That's gotta count for somethin'.