Acraig, I don't know if you have seen how it was solved in Morrowind the elders scrolls III, but I can't think of better system, if there comes about simplicity in use.
- The places where you can put items aren't limited, but appear as you have more and more items, so when you scroll, you will always see few empty slots. (and you are limited by your weight limit, in PS also by the capacity, which is visible in inventory window)
- segregation of items, you just drop item into inventory and everythink after the slot where the item was segregated move furher; or back, when you take an item.
- tabs for different groups of items
- the inventory field with items - resizable, like separate window.
Ah, when you pick or drop the item, it is happening instantly (not with lag), as the server probably has to check if you are allowed to. (It still check, but in the background, and move it back to original place if it decide it shouldn't be moved.
I hope i helped.