My un-informed non-insider two cents based on reliable media and FCA comments.

Most of the changes are guts like powertrain, engine, etc... a slight slope to the windshield, grill, and interior mods. The elongated front of this mule is probably because they are still using a current JK front end on a longer frame... remember JL will host a pickup truck version so the 4 door wrangler may be a little longer body.

 

There's been a lot of talk about the removable top not being totally removable. More like removable panels. No more soundbar. Can't remember where I read that.

 

The fact that they're going to build JK's and JL's together in the same plant for a short while lends credence to this. I wouldn't be surprised if things like door handles, mirrors, rear lights, etc... remain unchanged for the JL.