1. Instead of putting every single spell, potion, and scroll into the hotbar, have "Potion", "Scroll", and "Spell" icons that open menus with the appropriate type. That is, you mouse over the "Potion" icon and a vertical menu of just potions appears. For spells, I'd want a menu of spell levels, and then sub-menus with spells of that level. And not just the icon but icon and name. I haven't memorized all those icons yet, and it's a PITA hunting for the one I want.
Oh god please no. :-/
Optional or leave it to moders, but dont make us play with countless menus for every single thing we want to use. -_-
2. Find a reasonable way to prevent vendors from being looted of their magic items when you kill them.
If you are talking about scrolls, potions, and other stuff ...
It would be incredibly funny if attacked vendor would use them against you!
If you are talking about dissapearing weapons, that sounds pretty stupid. :-/
"Hey i have this +2 Greataxe ... but you have to buy it, otherwise it destory itself."

3. Speaking of cheesy, it's too easy to take long rests, which undermines resource management. Need some way to gate that.
This was mentioned countless times in countless topics ...
I would not expect tuning long rests until all classes and sub-classes are included.
4. Make it possible to keep weapons unsheathed. I cast Light on my weapon to aid in exploration, except then the weapon gets stowed and I lose most of the benefit.
I believe Tab is default button for unsheating weapons, you are totally able to run with them around ... not sure if you dont need to unshead them manualy after climbing tho.
5. Create alternate rewards (read: magic items) if you overcome obstacles in some way other than killing. There's very little incentive to not simply slaughter everything. It would increase replay value if there were a good reason (besides "I want to read the dialog") to try things different ways.
If you add magic items as rewards for other ways to deal with problems ... you would have as you say "little incentive to fight".

This dont sound like solution at all ... the story itself should be your reward. :P
6. Guidance cantrip: make it more obvious how it's working.
Agree...
There was one sugestion with alternated screenshot that i certainly liked ... it was 4sided dice next to our 20sided dice when we are rolling ... but im too lazy to search for it. :P
7. Sometimes I see that some of my characters have passed a Perception test, but then I can't figure out what it was they perceived.
Again, countless times mentioned ...
So far best idea seems to be different collor when alt is pressed.