GlassJaw
Hero
I personally don't see a big problem with a spellcaster using a reaction to counterspell, but if that is an issue, how about making it a reaction plus an action point?
If I was playing a caster and we were fighting an enemy caster, you can be damn sure that every one of my reactions would be used to counter that caster's spells.
Conversely, if I was DM'ing, I would also be using every once of my caster's reactions to counter the PC caster's spells.
So every round would turn into cast, counter, caster, counter.
That's not fun for anyone.
Requiring an AP might work but my gut feeling is that would make counterspelling used even less frequently than it is now.