Pros: Great support because of his blood boil
Has an amazing slow
His consume gives him lane sustain
Ultimate does tons of damage
Cons: His ult (if not used at good times) can be easily wasted
He is decently mana hungry

Nunu is such an amazing support because he can blood boil his teammates in team fights. Also his ulti is one of the best team fighting ults in the game. He is a strong champ that (if built correctly) can tank and deal a lot of damage. He is especially great when teamed with:
Caitlyn
Kog'Maw
Corki
Vayne
Nunu is an invaluable asset to any team.