Yorick -- one of the best bots, but still counterable by some champs. Banned because his spirits speed him up & slow you down, heal him up & damage you, kill his creeps while putting more in your way, and the spam never stops.
Urgot -- never see him banned nor picked, but i'm only up 30-something wins so maybe i'm diff elo. Banned (I would imagine) because of his high range poke.
Shaco -- always banned due to his ability to entrench, disrupt and defend. Banned because of his boxes.
Kassadin -- Tankiness, Burst, Kiting, Silence, Mobility -- He has it all. Banned because his ult lets him leap everywhere.
Rammus -- Tanky vs AD, Taunt vs AP, Powerball vs kiters. Good champ. Banned because of powerball speed.
Poppy -- great burst, mobility, tankiness -- Banned because his speed plus his ult makes him immune to disables.
Any good free week champ (like wukong right now, talon/singe last week, etc)
Anyone who just got a good buff (sivir was banned for awhile)
Ahri/Fiz/Talon (mage killers)
Kog/Vayne (tanky derp killers)
Heimer (a strong pusher on bottom)
Ryze (tanky mage)
Alistar (best tank)
The thing is, lots of champs are good, but you don't ban them because they won't be picked very often. You ban the POPULAR good champs, or champs that are good and not similar to anything else. As another poster said, why ban Talon if some other AD burst will be available to replace him. My answer is if I was fiddle I would ban Talon cause he silences me.