Taking a cue from ashockney (great idea!), what if counterspelling worked like this:
Counterspell (Feat) - A spellcaster with this feat may, as an immediate action, spend a spell slot of a level (equal to or higher than a spell being cast by an enemy) to unravel the enemy's spell. If he chooses to do so, he forfeits his next action. The spellcaster and his opponents must make opposed Concentration checks. If the casting character wins this opposed roll, the spell goes off as normal. If the counterspelling character wins, the spell fizzles with no effect. In either case, both characters spend the apropriate spell slots.