I'm not sure if she's overpowered, but I hate playing against Annie.
Her burst damage is off the charts. Incredibly high ratios and decent base values on three abilities. She also has surprisingly high sustained damage, on account of her low Q cooldown and tibbers.
She has counterplay, but it's very binary. The counterplay to annie is "stay out of her range at all times, or else you will get point-and-click stunned and bursted." That's all well and good if you're playing a champion with longer range than her. But there are relatively squishy champions who simply cannot do that, and they get totally screwed by her without much recourse.
I would be very happy if tibbers got the "corki Q" treatment (in exchange for buffs elsewhere, if needed). She'd still have a point and click stun, but at least tibbers would have a bit more counterplay.