He has MASSIVE cooldowns on his spells that he can only use ONCE in a team fight each, he has 2 attack spells that do decent damage early game, but have been nerfed repeatedly. He has major mana issues early game which makes it hard to harass without making yourself vulnerable

As a tank, alistar is definitely one of the best, but not op.
As AP, Play him before you call him op because the only thing he has is a knock up and knock back.

Not two stuns like leona, or amumu (whose CC lasts longer, and each second does about 2.7% of your hp in damage.)
And if you fight amumu for 5 seconds, there goes about 9% of your hp.