1 - All the icons and GUI seem a little sharper and cleaner. More distinguishable.
2 - THere's an extra story dungeon like the hospital that is repeatable. This is a story mission that lets you unlock the tech wing of your headquarters.
3 - The tech wing unlocks the sentry gun. So far it's kind of useless as it does very little damage. Also note that if you have it out in the dark zone and accidentally wing another player, your sentry will consider them a target even though you didn't go rogue.
4 - The Dark Zone NPCs are a lot tougher. I died so many times fighting one small group of them that they were respawning before I was able to kill them all. I did get a sweet set of gloves off the named one though.
I managed to hit level 8 within 3 hours by just doing the story quests and sub-quests. The last one is to go into the dark zone and I was 7 3/4. Doing that gave me enough xp to hit 8. There's supposed to be some new weapons etc, but i haven't seen anything yet.