I would agree the GTN does need some work, but it is far from the horrible mess some of you seem to be saying it is.
If you want to search by slot all you have to do is set your required tabs, hit search, and then fill in the relative term for the item your looking for. Slots of a certain level range all share the same name, ie Greaves, armguards, etc. Once you search based on that, you will get all of the items for a slot listed. Does it take effort to do this? Yes. Should it be neccesary? Not really, but that hardly makes it broken.
The most important thing you have to remember when using the GTN is to do one broad category search, and then search by name. Once you have that down, it's all simple.
As for why it's not an Auction Hall, that is fairly simple I imagine. Higher level players, ie Rich players, can't drive up the prices beyond unreasonable rates for the newer, lower level players.