Well, there are many reasons why you'd want to play NUNU, but there are a few key points I'd like to outline in this build:

He never gets nerfed!
Godlike sustain!
With new patch 17 ranks of your ultimate: with no cast time at all!
True damage Q (Consume) can be cast on your team mates to heal you, because when you have NUNU on your team do you even need team mates for support?

You don't need teamwork when you're NUNU, just remember that if your team comes to try and help in team fights, just use your Q (Consume) to take them out of the picture so you can 1v5 the enemy team successfully, without the potential for casualties in the form of your team mates!

Assuming he isn't banned (A problem fixed in the next PBE patch, as NUNU can no longer be banned in ranked picks), you just have to remember that they key to success is pumping your ulti as soon as you hit level two, making sure you have enough damage to 1v5 the enemies. If you don't do so, you'll find it incredibly hard to 1v5.

If you're playing NUNU in lane (Which is wrong anyway), then all you have to do is run back and forth, and spamming your Q (Consume) whenever it's up, which should be all the time assuming you're using the Rune setup I've mentioned above.

With NUNU, and his upcoming buffs, there should be absolutely no reason why you should lose a game ever again. With this setup I managed to get to 2.4K solo queue ELO in a matter of hours. It's not that hard to win with NUNU once you understand the team-kill mechanics of your Q (Consume).

Proof of my ELO (This was a few (Maybe 2 or 3) patches ago, but it still shows me at a good ELO using mainly NUNU):