Someone who has played DKS may be able to add more...
The damage for Explosive Arrows was reduced, and I think a couple other spells/skills were adjusted, as well.
It was the level dependence in combat that was reduced; AFAIK there were no changes to experience.
The equipment sets that were in D2:ED are in different locations, and more sets were added. Someone in the Atlus Ustream mentioned equipment only has level requirements, and not stat based requirements. Some dragon skill books have been moved or removed from the D2:ED locations (the librarian in Maxos temple no longer gives you one, despite still mentioning it in the conversation).
The effects and level requirements of some enchantments have been changed (generally higher effects and lower requirements), which are also used by the game's random equipment generator.
