I think the increased wheelbase is going to make parts like fenders and rock rails/running boards incompatible. We also know the hood and grille are longer/larger so there's another few that won't work. Hopefully JK bumpers and wheels/tires can be used.I'm sure the wheels will be the same bolt pattern and similar if not identical offset/backspacing.
My original guess was that most/all short arm suspensions will carry over, however, the rear LCAs are bent, similar to the front arms, vs being straight like the JK's (I wonder if they're actually interchangeable front to rear now...), so just right there there couldn't be 100% carry-over of a JK suspension kit.
I think front/rear bumpers will be interchangeable between JK and JL. I wouldn't be surprised if tube doors were interchangeable as well.
edit: And we've already seen that the rear trackbar will be different than the JK's, so that's another suspension component that wouldn't be a direct carry-over. In some photos it looks as if rocker guards could potentially carry over, both frame and body mounted, but then other photos it doesn't look like that would be the case...