Nunu is very strong indeed, one of my main supports.
He is always useful, no matter what you do.
If u had only the buff it would already deal such a lot of dmg and the slows are so nice to kite or protect!
But I wouldn't really say he's op.. just a very good pick
I've seen Thresh in some games already.. he seemed to have super good potential to become very useful once he's played right!