Short version: The Dark Knight has raised the bar for the comic book superhero genre - it's a Batman movie for grown-ups.
Screen Rant reviews The Dark Knight
It's a funny thing about watching a movie you've been anticipating so much and for so long - you walk in with preconceived notions of what it should be like. This happened for me with Iron Man, and as it turned out that film nailed my expectations of what an Iron Man movie should be like.
However that very same sense of anticipation hurt my first viewing of The Dark Knight.
Going in, while I was expecting an excellent film, I was also expecting something along the lines of Batman Begins - a superhero movie with more of a real world feel than your typical film based on a comic book character. That was actually one of the thithengs I really liked about Nolan's previous movie - it was close enough to reality that I could really imagine a guy dressing up in a bat-suit to fight crime as shown in the film.
But The Dark Knight takes that concept and multiplies it. You're not watching a superhero movie based in the real world here - you're watching the real world and it just happens to contain a psychopathic criminal and a modern day samurai.
The film opens with a bank heist perpetrated by a bunch of guys wearing clown masks, and what seems to be the Joker's initial arrival in Gotham City. The opening scene is brilliant and seems like it could have come right out of one of the "Bourne" movies. We also get insight into just how ruthless the Joker is as his henchmen have been instructed to kill each other after their particular tasks are complete (of course, they don't know that they've ALL been given this instruction).
In a further demonstration of how much the Joker doesn't care about or fear anything, the bank in question is where every mobster in Gotham City keeps their illegal cash - and that's what the Joker is stealing.
Batman and Lieutenant Gordon are more concerned with shutting down the gangsters by choking off their funds than dealing with "just one man." But oh, how wrong they are...
The Joker makes on offer to the heads of the crime families to kill Batman in exchange for HALF of their combined funds of $68 million. They scoff of course at first but soon enough they learn that they've gotten far more than they bargained for with our pasty-faced villain, who unleashes a reign of terror upon the city never before seen.
The Dark Knight is the closest of all the Batman films to one of the more mature reading level Batman graphic novels. This world is dead serious, and the Joker is no clown - he's a deadly, dangerous, semi-suicidal psychopath who also happens to be brilliant. Now there's a scary combination. Speaking of the Joker... Heath Ledger's death was tragic, but if there was a role to leave as a legacy - damn, this one is it.