You could go by descriptors... Say, have an at-will that cancels or reduces the damage dealt by any power with the Weapon descriptor, another that affected Fire/Cold/Thunder/Lightning/Acid effects, another that worked against Radiant/Necrotic abilities, and others like that.
Encounter and daily powers for the class, then, would either A) reduce the damage dealt by the attacks more, B) handle a wider array of descriptors, C) cancel powers based on action type, or D) a combination of the above. Most of these abilities would also deal damage, of course, since in 4E design virtually all powers do.
Unfortunately, I suspect that if you take this path, you're going to end up with a class that looks increasingly less like a Defender; as is, the Defenders all are tank-type characters with heavy armor and high hit points. If you want to take this class into the Defender role instead of Leader or Controller, you might consider giving it a wide array of abilities to redirect attacks to itself instead of countering them.
For instance, give them something like this as a class feature:
Once per round, when a creature you have marked uses an attack that does not include you as a target, you may choose to make yourself the target instead. Close powers must include you in the area of effect, while burst powers are centered on you if you are within range.
Special: You may use this power an additional time per round at levels 6, 11, 16, 21, and 26.
This gives you a much larger defensive and tanking role, since you can presumably mark a creature or creatures fairly easily, while still having a great deal of control over what enemies do. Heck, it might be better to have most of your "counterspell" abilities only work if enemies target you. This adds another layer of variety in powers that you can have, too, since you can have some of your encounters and dailies counter attacks that don't target you as a strong ability on their own.
That may solve another problem you would have, too: variety. Can't just have the same power over and over again for 30 levels, now can we?
Edit: Alright, I've got it. You have an ability, like the one above, that allows you to redirect the attacks of a marked enemy to yourself. One of your at-wills, one that all members of this class gets (similar to Eldritch Blast for Warlocks) counts as a ranged basic attack and marks its target. The other at-wills are immediate interrupts (maybe free actions) that trigger when you're the target of an attack, at which point you make an attack vs. your attacker's Will. If you hit, you can choose a new target within a certain number of squares... Let's say primary casting mod squares to give greater utility to it and more variance. Maybe secondary casting mod. The original attack is then compared against its new target(s) and works as normal. The differences between these at-wills will be, as mentioned before, what kinds of powers they effect. One will affect weapon attacks, one basic elemental types (Fire/Cold/Thunder/Lightning/Acid), and the third uncommon elemental types (Radiant/Necrotic/Poison/Force).
Your other powers will be very similar to your at-wills, except that they'll increase the enemy's attacks' area of effect, let you shape it to exclude your allies, increase the range you can retarget to, increase the damage they deal, add status effects... All sorts of fun stuff like that.
Take this suggestion, and I think you'll end up rather controller-ish, but that's not too big a deal. You can be a controllerish defender in the same way a paladin is a leaderish defender. Be a nice contrast.
...Man, you've almost got me wanting to write this class myself, but I've got too much else to deal with at the moment.