Buying a pair of socks I generally have my next trip in mind. Is it my annual end of the summer trip? or a weekend trek in the January snow? Lately I've been attracted to the Smartwool brand. Solid quality and fair prices and very comfortable. I've also had great success with Under Armor socks, but all things underarmor tends to be somewhat overpriced.
To me, it is the "feel" of the sock on your foot and in the shoe.
I generally use mid-weight SmartWool socks because they provide a springy cushion under my foot. Moreover, they retain most of that cushion even when wet, crusted with foot sweat and toe jam, and when saturated with trail dust and powdered horse manure. A quick rinse, a little drying time and they are ready for another 20 miles. They also last nearly forever.
There are other brands that provide a similar feel but I got a lot of SmartWool mid-weights on sale a year or so back.
I know that a lot of hikers prefer thin nylon socks; I don't and the above is why.
I prefer Thorlo light hikers for hot weather. They are comfortable, prevent blisters, handle sweat well, and seem to keep my feet cooler than wool sock. However, when the weather cools off, I switch to wool socks. I have smartwool light hikers, smartwool hikers, and the Sams Club brand. All of them are comfortable, but I actually like the Sams club socks the best. They are cheaper than Smartwool, if you can find them, wear really well, and for me are warmer because they deal with sweat better. The only negative is that they are not quite as soft as the Smartwool.
If I wouldn't eat it at home, why would I want to eat it on the trail?