Batman has looked similar but different in each of the live action films. yes, in all he's had a solid black rubber (was it made of rubber in BB? i can't remember) suit in each, but there has been differences. there's only so much you can do with a bat-suit. there's much more you can change/alter in the outfits of the bad guys, look at Burton's Penguin and Catwoman, Nolan's Scarecrow and Ra's, and, sadly, Schumaker's Mr Freeze, Riddler, Poison Ivy, and Two-Face. the villains are more based upon certain motifs than one singular look in the adaptation process.
i never said it was perfect, but i did say it was different. if Ledger showed up looking like every other live action version of Joker, i'd be pissed. i've seen that already. i want to see him performed differently. i want to see him look different.
Joker isn't tied down to just the purple suit really. he needs the crazy smile, and just because Ledger isn't smiling in this one dark, blurry image doesn't mean he won't smile in the movie. he needs green hair, dark sunken eyes, and red lips. the normal foundations for the character are all there.
it's just different from the norm. and that sends most fanboys into dizzying heights of overreaction.