I'd like to see some information about the value of an item in its tooltip or information page. In particular:
1. Shop value. The value that an item if you were to sell it to a shop.
2. GMS value. This one is a bit trickier, but even a somewhat inaccurate would be helpful. The lowest current(ish) sell order would be a good value. Or the average value of actual sales.
This would help with eyeballing the value of the many items that quickly pile up when mission or dungeoning. Of course, these both could be options in the option menu, for those that like to do things the hard way. This will hopefully prevent newer players from not looting (or selling to a shop) a valuable item, because it looks exactly like countless other low value items. A good example of this is instruction discs: most are worthless, but a few (Grid Armor) are very valuable.
http://i.imgur.com/KPXJc.png this is a screenshot of Eve Online's item valuation tooltip. It's based on the market, as there are no shops in Eve Online.
http://i.imgur.com/KhABgb4.jpg this is a screenshot of an addon doing a similar thing in WoW. Items in WoW have a vendor value in the tooltip by default, but an addon adds the market value.