It is?
Using magic to counter the spell of another spell caster as it's being cast? Sure. Maybe not in the term of Counterspell, but many other fantasy settings have Counterspell almost as often as they have a fireball type of spell.
It might be my exposure to other games that has imprinted me on the premise, but it's there. Legend of the Five Rings, Magic the Gathering and a few other editions of D&D.
That being said, I do see an inherit problem with counterspell. You do have to know the spell is in fact Arcane. And that does add a few other problems that is harder to address. I guess an Arcane check could be in order, but that really is very fiddy and not in the spirit of 4e.