The Library would be a great feature if you could copy stuff in it to the current character directly. But it would be a killer feature if you copy stuff in the current character to library. I've only been using HL for a few months and I can't count the times I've wanted to move a power in or out of an array or similar structure and the only way to do that is to delete it and recreate it in the new location. With a functional library, this would be much easier.
(Or have I missed some obvious piece of user interface that already does this?)