What if the action to counterspell was an unused attack of opportunity, rather than a readied action? Just a thought.
EDIT: Not to say that the opposing spellcaster has to be within melee range and provoking, just that, if a character has an attack of opportunity available, and an opposing spellcaster casts a spell that he identifies and is able to counter, that he can do that using said use of attack of opportunity.
… I hope that makes sense.
EDIT 2: or perhaps in addition to a readied action, so that a prepared caster could potentially counter both of his opponent's spells if he is on his A game.
The abjurer wizard with the counterspell sub-school can counterspell as an immediate action once per day. This is a limited and very specialized ability, and it still weaker than the possibility for any spellcaster to use (useless) attack of opportunity to replaces something that is actually a standard action.
However, counterspell isn't so powerfull (it uses spell slot to negate equivalent spell...when a good CC can negate the ennemy wizard spellcasting for 3,4,5 rounds).
Maybe a feat option could allow spellcaster to emulate the abjurer school ability and burn attack of opportunity to counterspell once per day, as an immediate action.
Or, an other option could be that counterspell as an attack of opportunity is available to everybody, anytime, but cost more (as a kind of quicken spell). The effective spell level required to counterspell could be 2 higher that the targeted spell and dispel magic attempt should receive a CL penalty (maybe -3 or -4).