This is a spot to continue discussion of counterspelling. I have suggested it be made a class ability that is an "attack of opportunity" triggered by an opponent casting within your range.
If he recognizes and knows the spell, countering is automatic as long as he uses a slot of the same or higher level.
In cases where the mage does not recognize or does not know the spell being cast he can still attempt it, using a DC of the opponent's spell DC + 5 - (counterspeller's level + ability bonus). Using a higher slot than needed reduces the DC by 1 per level higher.
Counterspell uses any spell slot the wizard has of the appropriate level. Even if he has the exact spell memorized, he does not have to lose that spell, he can choose another.
(These numbers are just a place to start. I have not done any analysis, this is on the fly)
A mage in a hurry could even not prepare spells in slots and still use them to counterspell (That would have to be a rough morning!)
OK, take your shots.