Why can you interrupt some channels but not others?

Malz: can be interrupted normally. QSS breaks the CC but not the damage or Malz's self-CC. (WW works the same way.)

Karthus: can be interrupted normally. Unless he's casting it while dead, since he's untargetable. Which is sort of the point of his kit.

Fiddlesticks: can be interrupted normally during the first portion of the ability while he's actually "channelling" (ie, before the teleport). Once he teleports, the "ability" has already been cast and is on cooldown. At that point he simply has a buff that gives the AOE damage aura. You'd need a "dispel"-type effect to get rid of that, and League doesn't have those.

Nunu: interrupting or killing him sets the ult off early for reduced damage. You cannot prevent the damage from occurring entirely once it has been cast.

Galio: can be interrupted if someone who is outside the area of effect CCs or kills him (or if an on-hit stun occurs or one of the AAs from the taunted champs manages to kill him). Like Nunu, the damage goes off early if he is interrupted or killed, and cannot be wholly prevented.

malzahar's can be interrupted if it is someone other than his target

karthus can be interrupted if he's killed (but not while he's dead) though it can be dodged while untargetable or in zhonya's

fiddlesticks goes on a short cooldown if interrupted

nunu's ult executes immediately if interrupted but at less than max damage, often minimal

galio's ult, I haven't played galio enough to know

miss fortune's is kind of silly because she can interrupt her own ult...same with malzahar

Fiddle needs to be interrupted before he blinks in.

Karthus can be stopped if alive.

Nunu gets stopped like MF.

Malz gets stopped if you aren't being ulted (also, QSS).

Galio gets stopped if you had an AA stun ready (Udyr E, Leona Q) or if someone not taunted CCs him.

lol, this type of thinking makes me want to see a nunu and galio ult wombo combo XD

nunu's is already at max 1200 + 2.5AP value , galios is max 720 + 1.08 AP value and whatever damage they might suffer from attacking him (thornmail)

if both have 300 ap, that comes to almost 3k magic damage, but the timing would have to be really good

miss fortune could be the ADC and Bullet time them XD

if only katarina could fit into that team, the damage would be completely nuts XD

it would also probably look like katarina were ice skating XD