Why do you say that?
IMAX has a special projector screen that's more advanced than the projectors at normal cinemas, which means you get a higher resolution than you normally would.
But yeah, the main difference is the IMAX screen is MASSIVE. The BFI IMAX in London is about 22 metres high and 27 metres wide -the idea is for it to take up your entire field of vision so that the screen is all you know. The sound is as big as the screen too are much more crips and louder than normal cinema sound system. Imagine DTS but amplified. It's an incredibly immersive experience.
Most films are just converted to IMAX, but where
The Dark Knight differs is that key sequences were shot using IMAX cameras which are 70 as opposed to the usual 35 mm used for most films. As a result of using IMAX technology, certain scenes appear FULL framed on the IMAX screen, making it larger than ever. And the picture quality is even sharper. Almost HD quality.
I loved that Nolan did that. He believes, and naturally, I agree, that the older you get the harder it is to take an individual out of the element when it comes to experiencing a film. The only way to combat that is a bigger, sharper screen with unparallel sound. Simple but so very effective.