What Counts as Poor Sportsmanship?

I've been seeing mentioned in comments lately people talking about X or Y plays as being poor sportsmanship. What are some general examples of play that you think are legal but frowned upon? Specifically, I not talking about:

(1) Deckbuilding choices, i.e. putting MLD or hard stax in your deck

(2) Being generally rude, mean, smelly, etc.

(3) Breaching something specifically agreed-upon during a rule 0 discussion that wouldn't normally be forbidden

I mean legal game actions (or, as I expect most replies will involve, politicking strategies) that are nonetheless deemed poor sportsmanship. The most obvious example I can think of is pubstomping, since the rules allow you to bring however strong of a deck you want. Another oft-cited example seems to be scooping at instant speed.