Actually,
* The Tractor beam array between the two globes on the tower was replaced with a communications array in the ISD2
* The quad turbolasers and ion cannons were replaced by eight-barreled turbolasers or ion cannons in barbette mountings
* The shields and armour were upgraded and the ISD had superior firepower
* Two grappling claws where added to the secondary docking bay for smaller vehicles
* ISD1 have triple baffles surrounding the main engines whereas the ISD2 has ring baffles
I don't know if that list is extensive or not
