From the most recent update:
Pillaging and razing are now options from the fort mode 'c' screen. You can bring back one piece of loot or one livestock critter per dwarf that returns from the raid (you can decide to turn either option off before they go.) Since we don't have actual production to replenish site stockpiles yet, we're handling it with a simple placeholder variable. The size of the raiding party vs. the current population of the site determines the depletion percentage, and the percentage repairs itself 1% per day if the site is inhabited. The percentage is the chance that an individual dwarf's loot action fails. So a ten dwarf attack on a small village can cause future pillaging to reduce returns or fail outright for months, while a 150 dwarf raid on a capital might cause a ~10% dip in returns for a few weeks. In the (distant) future, this can be replaced by numeric resource piles. Livestock, on the other hand, is actually taken from the site's animal population pools, regardless of the percentages.
On my first attack on a human village, the dwarves returned with a yak, a goose, and an alpaca, as well as a nicely decorated yak waterskin... and somebody's oaken crutch. When I sent a single dwarf to sneak into a goblin tower, she came back with a pair of silk goblin shoes. We might end up weighing them against picking up a bunch of junk after I try some larger raids, if the results are always that disappointing, he he he.
I suggest making it so both a dwarf's ability to find loot and what they choose to bring back depends on their skills and abilities. Possibly, the player could also specify loot priorities (like they do for trade agreements) which dwarves will respect or ignore depending on their personalities and traits; if that's implemented, it would be considered as a modifier for "value".
Specifically:
Preferences make a dwarf prefer to loot items, materials, and animals they like, possibly to the extent of ignoring value (or requests, if that's implemented.) The degree to which a dwarf's looting is governed by their preferences is determined by their other traits (like willpower and immoderation) as described below.
Observer and Intuition would help you find loot (increasing the pool of what the dwarf can choose from and decreasing the chance of finding nothing), and help find higher-value loot in general (under the assumption that higher-value loot tends to be better-hidden.)
Willpower helps a dwarf resist their preferences and choose higher-value loot even when their preferences would lead them towards low-value loot.
Analytical ability and Focus help with both.
Immoderation makes a dwarf more likely to choose loot based on their preferences, without considering its value.
Dutifulness and self-discipline make a dwarf more likely to consider value above their own preferences.
Additionally, immoderate dwarves or ones with high anger problems could do more damage to the site being looted, while compassionate and self-disciplined dwarves could do less. Damage to the site could make it harder to loot it again later, among other things - ie. they're smashing and destroying stuff that they're not taking. This could make diplomatic penalties for looting more severe - ie. sending a bunch of rowdy dwarves who kill half the cattle while stealing the other half is going to piss people off more.