Nunu's W now increases ability power by 40% (max 120 at rank 5)
Two issues with that:
- Nunu's effective (and I use the term very loosely) builds don't have any AP whatsoever.
- No AP ally of Nunu's should have only 300 AP by the time he's level 18 except for maybe Kayle.
Now, it may be that they only wanted this buff to effect when Nunu has a Kayle with Lethal Tempo on his team, but more likely they just don't want to buff him.
The buff should just be + flat AP. Making it 40% is completely arbitrary when the cap is both well above the 0 AP Nunu will have, and well below 40% for most AP builds his allies will have.
The only way it'd ever actually be 40% is if Nunu focuses the ability first, making it worthless for him because the max is less than the increase in base damage either of his other abilities gets from focusing them as intended, and making him worthless to his allies because his Q and E are his only real strengths.