Splatbooks, other than those that enhance the physical and political aspects of settings.
Games/settings with scores of monsters and numerous intelligent species, especially those which have no explanation as to how these races are interacting.
Endless lists of spells, talents, sub-classes, and the like.
Systems and settings which are just cookie-cutter copies of existing systems.
Fiction in RPG books. We get it, you wanted to be a real writer, but here's the simple fact: you're not. Plus your game system is going to get house-ruled and twisted into something you barely recognize, so your crappy short fiction will just be wasted space.
Trying to make your same seem unique by changing the names for GM, player, and other common RPG terms, including monsters. Call a vampire a vampire, Undead Undead, etc. I'm looking at you, Harn, but you're not the only one.