OK, don't know why I didn't think of this before: the different skill and inventory "tab/section" icons (e.g. "Warrior," "Fire Magic," "All," "Consumables," etc.) should be things that you can move with the mouse into empty toolbar/hotbar slots. Left-click-and-drag would probably interfere with the standard single-left-click selection of the icons within their respective menus, so maybe single-right-click would be the best way to "pick up" and move an icon into an empty slot initially. Then, once the icon is in the toolbar slot, you could simply single-left-click it to open that player's related skill/inventory submenu, or left-click-and-drag it to remove it from the toolbar/hotbar (i.e. just like any other specific skill/item icon in the toolbar/hotbar currently).

On a related note, for specific skills and items, single-left-clicking should be the way to "pick up" and move the individual skill or item into an empty toolbar/hotbar slot. This would avoid interfering with using items by single-right-clicking, and would also be in line with another suggestion in the OP: Left-clicking (rather than left-clicking-and-holding/dragging) on an item within the inventory window should allow you to pick it up.

The short version:
1) For individual skills and items: single-right-click item to use (if applicable); single-left-click skill/item to pick up and to move into toolbar/hotbar.
2) For categories/"tabs" of skills and items: single-left-click icon to select/open; single-right-click icon to move into toolbar/hotbar.

Thoughts?

Also, collecting some ideas for Larian's kind consideration from this and other threads which are not yet in the OP, or are edits to the OP (sorry if I missed any!):

1) After drinking from a container (bottle/cup/mug/flask/etc.), an empty container should be left in the player's inventory (NEW - posted by Raind)

2) The price for a stack of items in the trading window should be displayed both for one individual item and for the entire stack (EDIT TO OP - posted by Hassat Hunter)

3) A reassurance button/prompt should appear when the player attempts a trade without a balance of goods (posted by LordCrash)

4) The effects of both the player's Barter ability and the NPC's attitude on the purchase/sale price of each item should be shown in the item's trading tooltip (posted by Hassat Hunter)

5) The effects of a dialogue choice on a Trait should be displayed next to the choice (posted by Mikus etc.)

6) New NPC dialogue options not yet chosen by the player should be marked/flagged (NEW - posted by Dragomist)

7) Tooltips that still need more/complete/accurate details (i.e. correct percentage/numerical effects): Armour Specialist, Shield Specialist, Single-handed, Two-handed, Bow, Crossbow, Bodybuilding, Willpower, Charisma, Leadership, Lucky Charm, STR/DEX/INT and the three "Way of..."/five "magic school" abilities, Offense Rating (doesn't include effect of "Lucky Charm") (posted by Mikus etc.)

8) There should be two recipe tabs in the journal: the current one (listing the complete text of all recipe books the player has read), and a new one (listing specific recipe combinations in the format "A = B + C" for all items the player has actually successfully crafted) (posted by LordCrash)

9) A new dedicated crafting window in which items can be combined should be added (NEW - posted by Psykk/Stabbey etc.)

10) Single-left-clicking on an anvil should open a new menu in which to place equipment (posted by Fidelfc)

11) The player should be able to attempt to pickpocket each NPC repeatedly - there should be a higher chance of discovery on subsequent attempts, due to the NPC being aware he/she lost something earlier (NEW - posted by Gnoster)

12) After using a shovel, both your weapon and shield (if previously equipped) should be automatically re-equipped (posted by Mikus etc.)

13) A true "pause" function should be added - even opening the game menu currently doesn稚 pause the game; the game should auto-pause when a popup message appears (EDIT TO OP - posted by NeutroniumDragon)

14) Pathfinding should be improved such that party members will always avoid damage floors/environments (not only traps) - where a damage area can稚 be avoided (e.g. a tight corridor), the player should be required to manually control any party member who must walk through the damage area; a new formation "Follow the exact steps of the controlled character" could also help (EDIT TO OP - posted by Aramintai)

15) Character creation enhancements add 360-degree rotation of character models; add a true "classless" option (with no pre-assigned attributes/abilities/talents/skills, and with the ability to name the "class"); add the ability to change starting skills and equipment (EDIT TO OP - posted by LordCrash)


And thanks Hiver - if Larian wasn't putting so much effort into considering and implementing forum feedback, I wouldn't bother. But as it is, the ideas in threads like this are allowing us to make a good game even better.