The ignore feature is there for reasons. It gives us the option to avoid players we don't want to game with anymore. Used to be, the #1 way someone got on my ignore list was being rude in chat. I don't care if they're being rude to me or to someone else, and I'm not talking about constructive feedback. I'm talking about comments intended to make the other person feel bad about themselves. Not "You should take into account group comp when deciding how many casters to pull", but rather "You suk and should quit wow." Players making those kinds of comments to anyone go on my ignore list.
More recently though, I am ignoring people every week for placing troll orders through the crafting system. Not talking about Darkspear or Zandalari orders - Those are ok. Talking about orders with no mats and no tip. Or orders omitting the 3,000g mat and with only a 5g tip. The crafting table gives you the option to ignore those players, so you won't waste your time opening their orders ever again, and I add more of them to my list every week. I think there may be more on my list for this now than for rudeness in groups!
How about you? What's the #1 reason people get on your ignore list?