Ugh, it's the minor things that bother me. I don't mind at all when people wait weeks before answering trades, and I don't really even mind NOOB trades as long as they don't get too out-of-hand.
But I really do hate it when one takes the time to send a reasonable trade, and then it's rejected out of hand, not even a note like "sorry, but it's not for trade" or "I'm looking for something else, thank you for your time". Just someone pressing the cancel button and you're left thinking "ok, well, I wonder what I did wrong. Guess I can't ask that person for that pet again, even with a different offer". Talk about a stamp of "you're not welcome here, go away".