Just typed a book and it didn't go through, so I'll try to post high points and piggyback on an excellent post. Almost all of my playtime is Online Co-op, some of the points wouldn't serve anything in Single Player

1. Need to be able to Ping or add Markers. Would let you plan or point out things much easier (not talking the markers on main map, think MMO markers on where to stand etc)
2. Inspect Buddy - Aggravating to not see builds, stats, perks, armors. I never am the Loremaster so idk if that is what's needed here or not
3. Follow Command / Shared XP. Quests/kills should apply for everybody, I'd love to be able to split up for more mundane tasks. (i.e. you free Amyro, I'll do Migo) but so far when we do this (and sometimes when we don't) the characters end up on different levels at times. Follow is self explanatory, would love to get a drink or take a call and not try to catch back up or slow party down for 5 mins.
4. Would love to see some way to simultaneously shop at the same guy, maybe not be able to seal the deal but at least view inventory, talk with party members, etc. It's tedious and aggravating now, especially if you have ur "barterer" do all the deals.
5. "Party Stash" I like the magic pockets, but I'd also like to be able to send all my superfluous stuff to a party area. I pick up everything, but don't really wanna have 100s of shells and plants in my inventory. Or have to send some stuff to a friend because he needs 20 gold and I'm in the middle of something as well.

Stuff that seems broken:
Teleport and "Slow" spells are crazy strong in my playthroughs. (Slow is probably the most dangerous of all in my solo runs so far)
Hunter builds seem to have 0 drawbacks and do as much or more damage than almost every class
Leather Armor seems like it's better than Cloth or Plate to me regardless of role. If I outfit my Tank in all Plate armor It seems like I'm running 100 p armor to <30 m armor (Mostly from shield).
Memory I love and hate, I run a lot of spells generally, and so most of the time I end up with 15+ wits n' memory, very little left for the rest. This may fix itself at higher levels when you have all the spells you need. But seems like it needs tweaking to me
Most of the OP's other stuff (shared barter/lucky find) etc.