A lot of the hype is coming from the fact that Heath Ledger died not long before it was released, which often makes people wary of criticising what is rapidly becoming 'his' movie, as they fear being labelled 'disrespectful of the dead', which is a nasty, sticky label.
I won't deny that Ledger's Joker is awesome - that is a fact. However, I can tell you that, overall, the movie was good, but not to the levels of truly spectacular movies out there. It had good parts, bad parts and awesome parts, in roughly equal measure.