I have certain pet peeves when it comes to attempts to define science fiction. Here are a few:
1) Definitions that aim to exclude science fiction writers or subgenres you don't appreciate;
2) Definitions that aim to exclude your own works from science fiction just because you don't want to be labelled as a science fiction writer;
3) Definitions that seek to elevate science ficion over other non-elite genres; e.g., "science fiction is that branch of fantasy which does not suck (because it's so much more, like, rational, progressive, logical, systematic, relevant, etc.)";
4) Definitions that attempt to prescribe what future science fiction writers should attempt to do.
I have a few more almost peeves, but these are less about definitions than about origins and lineages, so I'll save those for another post.
What are your peeves?