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In 3.x you could just ready an action with Dispel Magic (or Greater Dispel Magic, or similar spells) and make a Dispel check to try to counter the spell. 
A Dispel check was  (1d20 + your caster level, maximum +10 for Dispel Magic or +20 for Greater Dispel Magic)

In D&D Next they broke up Dispel Magic and made it simpler by cutting away the counterspell part and making it a new spell of lower level.

You could also counterspell by casting the same spell if you knew it (and had to make a check to understand which spell was cast)

Now it's pretty straightforward and I like how it works...doesn't even take an action, you could counterspell AND cast in the same round!
...but honestly I would never prepare it unless in very specific situations... I'd rather have Dispel Magic as it was in 3.x, much more versatile.

To be a class feature, you should be limiting it. With a 2nd spell slot you can only counterspell spells of 3rd level or lower (the enemy consider it cast) or prevent other spells (even a 10level spell!) from working, with just spell of 2nd level.

I'd make it this way. I wouldn't give it at 1st level, otherwise it would be too easy for non-caster multiclass characters to hinder ALL spells by just taking a level of Wizard.

Level 3: Counterspell (class feature): By preparing an action, you can target a spell cast whithin 50 feet of you. You and the caster make opposed spell attacks (1d20+magic attack bonus+magic ability modifier). If you win, the caster's spell fails but is not expended.
Level 6: Improved Counterspell (class feature): just before making a counterspell check to prevent someone from casting or to resist a counterspell, you may consume a spell slot to gain a bonus on the check equal to the level of the expended slot.
Level 12: Superior Counterspell (class feature): when you use the counterspell class feature you may expend a spell slot before making the check. If you win and the level of the slot you used was equal or higher than the spell you tried to counter, the enemy also expendes the slot. If you fail, you get +4 bonus on counterspelling checks for 1 minute.
Level 15: Counterspell Mastery (class feature): when you ready an action to counterspell or win a check against counterspelling you may also use a reaction to counter another spell cast before the end of your next round. 

Well, I only need level 1, 9 and 18 and it's almost a specialty :D (maybe +1, +2, +3 to the check?)

@Drahiricomadus, I like your proposal. 

What if a successful use does minimal damage the wizard countering the spell? Like they are absorbing the spell's power and it weakens them each time. This should encourage players to reserve it for emergencies.

Also, make it so they can only counter spells cast on allies. If an evil mage transforms himself into a crow or empowers his skeletal horde I don't think that kind of spell should be countered. But if the mage uses an attack spell or tries to control an ally's mind then the wizard can sense that something in their arcane aura is changing, or some such. The wizard in attuned to his allies normal levels of magic/non-magic.

Sep 18, 2012 -- 4:14PM, EnglishLanguage wrote:

Or "Damage that cannot be reduced in any way." to the Wizard counterpspelling equal to the Spell Slot counterspelled....just tossing it out there.

Nice idea. Maybe which ever is higher?