Here are my thoughts
1) For GTN to be useful, it has to be an effective friction-free market. So definately needs to be cross-server for faction at least. Understand you will have more competitors but you will have just as many (if not more buyers). Free market will server Both buyers and sellers best.
2) Provide tools that make the work centered on a) identifying a vialble market that needs good (e..g grade 5 purple gemstones), allow us to find what fair price is (i.e. log our sales, allow us to watch similar sales and give us recent sale price history on GTN).
3) Allow buyers to post needs (items wanted tab)... maybe there is a level 20 sentinel which wants purple enhancements... how do you alert aritfice skilled players of your desire (at specified price).
4) Fix bugs... duhj
Bottom line, seems like sometimes its just easier to grind missions, PVP, etc to get what you want than just purchase on GTN (and to be honest some items proably should require that...and conversly some items maybe should be crew skill only).. If Bioware wants people to invest the credits in learning craft, and spending credits on mat gathering... there has to be reward. A tool that gives largest friction free market, with powerful price researching tools (is it seriously required that I write this stuff in pen and paper) and valuable crafting items should be goal.