I hate my socks. Maybe it is the size of my foot that I dislike.
Socks come in size ranges (i.e. 10-12) and the size of my foot tends to fall in the lower half of the range that fits me. Because of that, the reinforced heel ends up behind my achilles and it looks stupid. It looks worse with athletic socks, because they tend to make the reinforced areas grey. It's a small thing, but it annoys me.