I'll start off with a couple:
Blatant inconsistency within group builds. I'm not talking about something being just slightly off, that's fine. But when someone builds in a city that's of a realistic, modern style, mainly 5 blocks per storey, and makes a wood and coal single-storey house that's 12 blocks high, that just grinds my gears.
When people say "wanna be friends?" (self explanatory)