So I have a bit of an issue, and something I feel needs fixed. Kassadin's silence seems to be a little strong. While playing Nunu and channeling his level 6, I was silenced. It completely canceled the spell, and still put it on CD.

I think this is a little ridiculous and needs to be addressed. Either prevent the silence from taking effect after the spell has been cast, or if it stops the spell leave it off CD. For it to stop the spell cold and put it on CD and making it unusable is just insane.

Silence shouldn't interrupt? /what logic is that? Whenever I played Kassadin it seemed half the time the silence didn't even work.

It does not work on channeled spells, like Fiddle's ulti when it's up, or the Lich's (I think it's "e" skill") or Anivia's ulti. THAT is kind stupid and should be fixed. Kassadin's "q" spell isn't that great, it's just useful to prevent heroes from casting, but there is way more efficient cc to achieve that, like stuns or fear.

Wait, so it silences cast spells in progress but not channeled spells?
That seems backwards to what I would expect it to do.

The thing I found annoying is it silences champions like Mundo from using his special abilities. I think only abilities that require mana should be silenced.

I think it should silence all spells at any time they will be or are currently being cast.
I don't think silence should prevent non-magic champions, like Mundo and Trynd, from using their abilities.

I'm assuming your issue is that you like Nunu and don't like it when Kassadin silences you when you get the Ulti going. However, Fiddlesticks, Jax, Twisted Fate, Alistar, Cho'gath, Blitzcrank, Sion, Taric, Warwick, Anivia, Annie, Veigar, Ashe and maybe a couple others have a way to interrupt Nunu's ulti either through silence, stun or forced move (In the air or across the screen). All of which would put Nunu's ulti on cooldown.