Just a simple but huge fix I think. I'm sure it's not so simple on the back end, but on the front end, it would help a lot of issues I have with the game.
I am constantly going back to my Inventory to see how many items I own. Mostly this falls prey to two professions: Crafting and Mining. Both of which are integral.
With crafting, I never know how many bars I have for a craft. I have to scroll through my recipes to get clues instead, or go to my inventory (which I can't search by the way, I only have icons…) and try to locate the item, by icon. Mousing over them to check my item count felt intuitive (Maybe because I'm an MMO player, I don't know if anyone else instinctively tried this).
Especially with the new numbers update, squishing extra numbers into K and M, the tooltip wouldn't risk a bloat issue with other areas of the game (like Bones…).