Cynwrig! I'm really glad to see you continuing this. Your thread here has been a big inspiration to me—Arianna and I have always wanted to improve the GUI, and this has really encouraged me to work on it. So many great ideas, mockups, and good discussion.
I started implementing your group and target windows, and hope to work on a lot more UI after this upcoming release. I would really like to see many of these ideas implemented, and I think we can make it happen.
So now for some comments!
Inventory WindowIf you don't know, the inventory subsystem rewrite back in 0.3.019 was one of my first huge projects, so I'm pretty painfully aware both of issues surrounding it and also how
hard it is to implement. We simplified it a lot by disallowing nested containers, and this is even better.
I really like bulk inventory becoming your "backpack"—presumably everyone has such a container, and it cannot be dropped or sold. The tabs for containers are wonderful. For clarification, in the new design, other containers like Sack 1/2 don't appear in slots in your backpack—
only as tabs? Presumably you could drop containers by dragging the tab outside the window, just like items.
One problem I foresee is moving items from one container to another—you can only see one tab at a time. While there are several approaches one could take, the one I like best is to add a "shelf," a concept I think I'm borrowing from the NeXTSTEP UI:

(it probably wouldn't have to be that big, but we can play with the layout)
The idea is that the shelf is "temporary storage"—if you want to move an item from one container to another, you drag it to the shelf, switch tabs, and drag it back.
You could put as many items as you want in the shelf—so if you want to move everything around, you could just stick a bunch of things there, and then move them back out.
Naturally, to avoid infinite storage, you wouldn't be able to move/fight/cast spells/craft while you had something on the shelf—just like overweight now. You'd have to put the offending items in real slots, first.
Inventory is incredibly difficult to code, and one of the hardest places is handling failure. Before I added the "overweight" system, we had to handle failure when we didn't have the weight capacity to carry an item. Imagine trying to handle this halfway through an exchange, or when getting a quest item, or...who knows. You
really don't want to hit that case and risk losing your quest item or completely botching the exchange. My clever trick was to make weight never fail us—we allow the operation anyway, but set the player "overweight" and let them sort it out themselves. Humans always do this better than a program.
The part I love about the shelf, aside from being a natural solution to the container-switching problem, is that it solves the "out of slots" problem too! Get a quest item, but didn't realize you were out of places to put it? No problem! Into the shelf it goes—and once secure in your possession, you can find something unimportant to drop. This has the potential to clean up not only the UI, but one of our ugly problems in the server code.