I'm sure this has probably been mentioned before, but the Trade Network interface really needs work.
I find myself vendoring everything just because the hassle of the GTN is too much hassle. I'd like to be able to search the name of an item without having to enter in every category first.
And I'd like to be able to search for just an item type, without having to enter the subcategories first.
Sometimes you find great stuff by browsing randomly, without knowing specifically what you want, and it's easier to sell if you can just look around without having to use to item category and subcategory.
Just my two cents.