Fair points.

I think restricting this to non-creature spells is an entirely reasonable suggestion, and one I plan to implement.

The card originally countered, but I shifted it to exile because I wanted it to unconditionally "counter" spells. As for cost reduction for abilities, I specifically didn't include language to reduce the cost in the event an ability was the intended target for the counter. Abilities should be harder to counter, in my opinion.

I think, after the feedback here, I'll likely change it to 4UWW.

A point I'd like to make, however, is that I think many in this thread are failing to take into account the context of the decks in which this would be played, and which decks it would be played against.
If this is played in a UW control list against Jund or Junk or Burn or Lantern or Infect, the cost reduction will only ever be between 1 and 3. Does this get better in control mirrors? Yes. But the concern that this will be consistently run out for two or three mana is, I think, unfounded.