I dunno, maybe its just me, but do any of you vets dwarfs really need to trade at all? If anything, its more of a nuisance interrupting my otherwise busy work in fort construction, and generally I just do it because, well, its there. Why not grab a few gems or spider silk or whatnot just in case there is a wierd mood, but nothing really out of necessity. Course, I usually dont play maps yet that are devoid of metals extra !fun!, so what do I know?
When the fort first starts out, I'll build up enough crafts and other trade goods to buy out barrels and booze. It takes a while to get the farming/brewing industry up and running, so I always play better safe than sorry when it comes to plentiful booze. Often I'll also have to ask for wood, at least until I breach caverns and hopefully find things to cut down if I've embarked where there are few trees. Initially I'll also buy out some cloth and leather in case of strange moods.
Once the fort is established, I'll often ask for interesting animals and gypsum plaster from the dorf caravan. I don't know that I've ever embarked anywhere that I could actually make gypsum plaster. (Can you even make it?) Also, again depending on where I've embarked, more wood.
Humans always bring tons of food for me to make more lavish meals which I use to trade for more food. (As well as plenty of raw materials for training chefs and making my dorfs happy with decadent meals.) I always try to embark where there is metal, but if not, they usually bring lots of armor and weapons I can melt down.
Elves... elves are useless. Except when occasionally they bring interesting animals, which is almost never. I have gotten breeding pairs of grizzly bears or badgers from them on occasion. So I'll usually check out what they have before slaughtering them mercilessly. which, sadly is still kind of useless, because they won't even siege me.