I think the NADs at the moment are quite decent, maybe not perfect put still works to a good extend if we take into account all abilities. I think too many players try to only do damage and forget about statuses/defenses.
I'll give an example, currently I have 2 groups running the same campaign (heroic tier atm) doing the same mission the first group got hit a load, and the other were able to beat most of the groups barely taking any damage.
The difference was how the defender in each group was built, the first defender didn't take the feat or powers to do full out defending whereas the 2nd group had a defender that did nothing but that. In numeric terms:
Baseline hit for monsters: rolling 8s to hit PCs
Mark: -2 to hit
Feats: (+1 def) Varying but at level 3 was able to boost defense by +1 for anything the defender was marking.
Extras: People using powers to up the defenses or using second winds/total defense.
Total: 11+ on NADs (13-16 if powers came into play)
This was only with one feat on the fighter, using mutlitarget hits to mark more targets. At later levels you have even more tools to be able to lower attacks again.
For example a paladin + rattling rogue, again numeric example:
Assuming hit for paragon tier on NADs: 7
Mark: -2 to hit
Rogue: -2 to hit
Powers: -2 to hit (varying but you can pull this off with an at-will)
Total: 13+ on NADs (this can increase depending on other powers/feats used to boost defenses)
So no feats in play here, paragon tier synergy between to party members to make it more difficult for the monsters. I have often found that if there is a good defender/controller/supporter (striker helping debuff) then the party doesn't get hit that much.
Of course there is still the problem that there is always members with a lower defense than the others except maybe for the defenders, but I think this was inbuilt because even the monsters have one lower defense than the others and the DMG states this. The DMG also says not to tell your players which defense is the lowest for the monster.
Summary: In my opinion the design intent was to make the defenders let the to-hits drop to about the 50% line then feats/other powers to drop the hit to 25-40%. Although as I stated before it ain't perfect some tweaks could be done, I'm just not sure of the best way.