What annoys me is the Always Chaotic Evil trope, in which every member of a race is stereotyped into exactly the same cookie-cutter format.
Sometimes it's justified. Brandon Sanderson, for instance, has perfectly good reasons for portraying every last koloss as a mindless savage beast. When done right, it's fascinating to see plausible explanations for why every member of a species might display the exact same behavior. But more often, it's done only so a writer can summon up endless hordes of endless minions for adventurers to slaughter.
Case in point: kobolds in Dungeons and Dragons and Dwarf Fortress game universes. They are portrayed as skulking, evil thieves in all appearances, which is held to justify acts of random murder and genocide whenever "heroes" come across their villages.