Caitlin's trap.
Any other trap can be set off as soon as it's set. So how come Teemo can poison you while you're trying to run, but Caitlin, who's trap does next to no damage has a year-long time between when she sets it and when people can set it off? That's a little bogus. I've been trying to learn her and it turns out she simply has two moves that can't be used during combat and one that has a cast time longer than Veigar's E, Karthus's Q and Brand's W.
I think her Q needs to be thinner but lose the cast time. It's always bothered me that some wide thing comes out of her sniper. It can keep the pass-through decrease, but lowering the width would help it hit fewer minions. As far as the other things go... II simply don't like the style. Sure, she has a lot of range, but why does that have to mean her damaging ability takes the time of two basic attacks (that will probably do more damage) to cast, and the other two require your enemy to not walk on top of something clearly visible or be running away extremely close to you?