She's the hard counter to stacking HP. Everyone is stacking hp. In draft, if your opponent takes her, you can't take her. She will be stacking hp.
Simple as that. People can just...not stack hp. Though even without that, if she's selected as your top's counter pick you're in trouble. Some tops literally have no chance whatsoever in a "you're vayne early game against Urgot" kindda way.
She's been a top ban for like, 2 months*. She's been dominating the European and Asian tournament scenes basically since Season 3 started. She has % health based poke with great range on a 6 second cooldown, she has an execute ability, she has a stun, she has 2 escapes, she has wave clear, she can build full tanky and still do damage, she has naturally high movement speed and armor/mres, she even has an attack speed buff to help her push down towers.
She's an absolute monster in a meta where stacking health is the best way to win. She doesn't lose any lanes and only stalemates against Kayle, Jayce, and possibly against Jax or Akali if they get lucky with creep spread before they hit 6.
*http://www.lolking.net/charts?region=all&league=ranked&type=bans&range=monthly I wish there was a way to go back more than one month in LoL King. There probably is, actually, but I don't know it :<