For me... it's either:
A) guys who flip every play (sometimes multiple times per play) because they've seen the "MCS Pros" doing it so they figured it must be a good thing to do.... or
B) sore losers who as soon as they realize they're outmatched, decide to make you waste your time by taking all their pauses and timeouts and grief offsides... etc.
As for "A"... I sometimes flip my play... if I decide I want to take advantage of the short or wide side of the field. But I think most players who do this a lot are looking to take advantage of bad coding in the game. For example... if you're in Big Nickel... and you flip your defense because the offense has flippd their play... your deep Safeties sometimes do this weird criss-cross of assignments. I'm sure other things get messed up as well.
As for "B"... these D-bags are reserved a spot on the inner circle of hell (only guys worse are the freeze glitchers).
What bothers you most about your worst opponents?