I beleive Riot actually admitted at one point Blitz is OP, but nobody uses him so nobody talks about him. I'm pretty sure Kayle is finally getting her rework next patch.
Galio - Tank that can only tank with his ult. His Q is okay, his W sucks, and his E sucks. His R is easily countered, and mercs allows people to just walk away from it.
Veiger - Has to farm all game to do much of anything, and even then he just nukes one guy and is useless again.
Nasus - Has to farm all game to do much of anything, has no gap closers and is easily kited. Also his E flat out sucks.
Sivir - Needs a massive rework IMO. A few number tweaks won't do it.
Pantheon - Needs a massive rework IMO. A few number tweaks won't do it.
Alistar - His stunlock combo is way too strong. His R lets him build AP and still be able to tank during teamfights. Now a top ban in many games.
Warwick - He was always strong, and now that Nunu has been nerfed he's quickly becoming a top pick every single game. Too much sustainability, stupidly low skillcap to do well with him.
Annie - Was overbuffed, nothing more needs to be said.
Morde - Needs a rework. Almost as mindless as Sona to use, and has a stupidly strong laning phase. As said, he stomps and forces early surrenders or becomes useless late game. Eve got nerfed because her impact on a game is anti-fun. Morde does the same, often forcing people back to their towers all lane phase.
Soraka/Janna/Sona/Taric - People will flame hard for this I'm sure, but when these four can have a major impact on the game without building anything but boots there is something seriously wrong with their skillsets. No champ in the game should be able to give 100% of their farm away and still do well. No one. Nerf base ability stats, improve scaling. They should have to build AP like Karma and Kayle. Zilean is okay, as he completely falls off late game if he didn't build some AP.