Modern mores and social norms in pre-modern societies. Basically modern middle-class North Americans in ren faire clothings. Paizo's stuff is the worst for this.
Settlement patterns that are closer to the American West in density than medieval Europe, often with 15 or 20 miles between settlements. In medieval France of England, a 30 mile by 30 mile area would have more than a dozen villages, several towns, and a city or two. If your settlements is two days walk from the next closest settlement, you've got a god-forsaken backwater, not a bustling medieval town with taverns and other signs of healthy commerce.
As mentioned above, the superficial trappings of feudal society without the social structure.
Lazy real-world analogues (fantasy-Arabia over here, fantasy-Russia over there, fantasy-Aztecs down there, etc) instead of original cultures.
Humanoids as pests rather than horrifying monsters. Goblins should do far worse than steal an occasional sheep. Humanoids should have a niche distinct from small and ugly bandits.
Nations of good people worship good gods and nations of evil people worship evil gods. A religious pantheon, and the culture that sustains it, should accommodate the full sweep of archetypes and deities.
Far too much focus on high-level politics and history, and far too little on boots-on-the-ground geography, lairs, ruins, and NPCs.