Second Life Insider is just asking, how would you fix the SL client UI? One of the answers I very much agree with is: fix the inventory.
One thing I wish for may have to do with my cluttered inventory: I'd like to have two windows, perhaps like the old two-window file managers. It's a huge pain to have to scroll huge distances from where you've picked some items to the folder you want them in.
Another thing, though, would be great even for the well-organized. Inventory needs to get away from having each item in exactly one place. In Linux/Unix terms, it needs "symbolic links."
It means that inventory isn't a tree; things can appear in more than one place.
Why do you want that? Because a major use of folders is for outfits, and if you mix and match, some items are part of more than one outfit. As inventory is now, you either copy them, or you don't copy them, in which case you have to track them down all the time.
With symbolic links, you're not copying the object--you're just providing more than one way to get to it.
UPDATE: Thanks to bcwhittle for pointing out that you can open two inventory windows! (See the comment.)