Simple: GO SEE IT.

Why you ask? Well, here's my logical argument:

1. The Joker returns to the big screen for the 1st time in 19 years: Without the 1989 Batman, there wouldn't be Spiderman, X-Men, Fantastic Four, Iron Man, and various others. The 1989 Batman set the standards for a faithful comic book adaptation, almost calling for a deviation from Adam West's variety of campy action, and a return to a gritty, dark, type of superhero. And the Joker helped to make this all possible.
2. DC Comics's only summer film: Basically, if you're a DC devotee who likes how they treat their film franchises (unlike Marvel, who after Spider-Man, went crazy and green-lit the whole damn universe), you'll be watching The Dark Knight.
3. Batman Begins = Greatest reboot in the past few years: Obviously, Batman has had his fair share of crappy media (i.e. anything from the 60's and 70's, Batman Forever, Batman & Robin, etc.), and if Begins had remained in the vein of those awful campy films, one would have to wonder if Batman was unable to conquer Hollywood. Thankfully, Christopher Nolan and Christian Bale stepped up and gave up Batman Begins, therefore erasing the franchise's awful past and starting over.
4. The Dead Guy Factor: Yeah, just like music, where someone dying brings that person right back into popular consciousnesses, this film has that extra intrigue in in. I've seen the movie and let me tell you, the "dead guy" factor is non existent. Ledger's performance was awesome, and for me, it was the Joker portrayal that finally edged out Mark Hamill's superb version in the 90's Animated Batman.
5. Summer movies this good are near impossible: Let's face it, summer movies are slowly losing their magic feeling, especially when some put 'em out every single summer. But this one is so much more....

If my argument hasn't swayed you, then nothing will.