Why do you not play support Nunu instead? >_> Both operate almost the same way except Nunu loses the armor shred of magic circle for attack speed and movement speed buffing on his ADC. Not to mention Nunu also scales much harder with nominal AP items than Nasus does.
The worst thing I would think is that instead of being instant the Ice Blast from Nunu has to travel to hit whereas Nasus Wither is instant so counter initiating with it is much easier than with Nunu. --- http://oi41.tinypic.com/9ht4ea.jpg No matter which way you slice it you'll both be ending up in Hell!