Some of the following have already been mentioned but here is what I feel would greatly improve the game.
#1 - Option to open all bags at once
As the title suggests, I'd like the option to open all of my bags at once.
#2 - Quick Transfer
I'd like the option to use a key command + click to quickly transfer things from my bag to my stash and vise versa.
#3 - Invert camera
I'd like the option to invert A + D to pan the camera.
#4 - Mining/Pick Action
It's annoying that when mining, after every attempt, the character puts his/her pick away and has to get it out again.
#5 - Bigger stacks (Popular request)
Already mentioned many times. Ore needs to go into bigger stacks then 10. 20 would be good.
#6 - Hovering over an item
I'd like to be able to hover over an item and for it to count the quantity of the item I have in all my bags and tell me what it is. So if I have 10 Copper Ore + 10 Copper Ore + 3 Copper Ore in another bag it would tell me I have 23 in total.
#7 - You're -> I'm
When your character is speaking to his/herself, it should really be "I'm hitting bedrock" not "You're hitting bedrock". Who is telling me I'm hitting bedrock?
#8 - Home stone cooldown
Hovering over your home stone should tell you the cool down. You shouldn't have to attempt to use it to see if you can yet and it would be nice to know when it's due to be ready.
#9 - Pets can't keep up
My NCPet can't keep up with my character. It often lags behind and as a result despawns.
#10 - Whispers
I feel whispers need their own chat tab.
#11 - Chat width
On larger screens, the option to extend the width of the chat window would be a nice addition.
#12 - Quest log
The quest log is pretty complicated I feel. I feel it should have Main Quest, Side Quest and Craft Quest tabs. Presently, I've found myself stuck in a loop doing crafting quests until it was pointed out that I won't progress the game doing them now.
Sounds like a lot but all of the above (coming from a new player) would make the game much better in my opinion.