Truth be told, I was thinking the exact same thing. It would be a steamroller.
I know!!!
I've always wanted a all defender team in general, but yeah. that would be amazing and unstoppable. would totally beat a team of blasters. 
not to derail, here I go anyways... I know Repeat Offenders was still popular, but most people in CoH play dominators, controllers, brutes, scrappers or tanks for iTrials, my obsevation.
All defender teams rock. We did some all NA teaming Tuesday (our little play group) on Infinity but it was not limited to Defenders. Defenders, Corruptors, even my mastermind representative rocked the lowbie content.
Triumphant Defenders Forever
Psylenz FF/Psi, ArticQuark Storm/Rad, Symon BarSisyphus Bots/psn, Max VanSydow Thugs/Dk, Cyclone Symon Bots/stm, Blue Loki Ice/Cd, Widow 46526
HelinCarnate:OMG it is so terrible. I have the option to take 3 more powers but no additional slots. Boo F'ing hoo.
Yeah, I think if you're going to get a superteam based on NA to stick together you'd want to allow for everybody who can use it to join. I find the more restrictive a superteam is the more likely it is to fall apart when the lead pack gets into the upper levels and starts wanting to do higher level task forces.
The only problem with an all Defender NA/ group would be the damage cap. You would easily go over it at level 32+.
On the other hand, doing a Posi yesterday with 4 NA chars. I swear I saw the EB we fought hit for 3pts of damage.
Good point on the damage cap. However there is some significant -resist in the tier 1 power alone, so that may help quite a bit. Obviously MM's and corr's pump out more damage. A good mix NA AT's with minimal slotting would probably still be a steamroller. If everyone got and slotted maneuvers as well (as most Superteams too), it would/could be at softcap.
The other issue is that with the +to hit, the def's/corrs would all have instasnipes being well over the 22%to hit.
The mechanics is weird still, such as angling heals and such. It would never be as potent as the FRAD's or Brutal Speed, but I think it would still be interesting.
I think I will need to level up my NA and start an all def NA ITF or something for kicks.
My work and travel is not conducive to starting this, but look at the def numbers and say.....hmmmmm, this is some crazy buffs/debuffs. Now multiply those X 8. Drool.
NA/Fire or Ice ST? If you build it, they will come.