Your player characters need better NADs.
But seriously: I wonder if the designers took this sort of problem into account. I would like to think they didn't give most mosters an At-Will attack that practically always hits by targeting the PCs NADs. If they goofed this up, then DMs shouldn't always be using the easy attack/dirty trick of hit the PCs in the NADs.
It's a bit below the belt, don't you think?
I don't know that the designers did take this into account. For all that 4th edition is supposed to be mathematically balanced, there are a lot of mathematical oddities (skill challenges being the most egregious example).
AC defenses will potentially improve by 15 (level) 6 (masterwork--or 2 masterwork and up to 5 attribute), 1 (feat), 1 (paragon path) and 6 (enhancement) over 29 levels for a total increase of +26-+30 depending upon the character's build. So, it is possible for a 30th level character to have an AC that is effectively one point better at level 30 than it was at level 1. Practically, however, most characters will be slightly more likely to be hit by monsters at level 30 than they were at level 1.
Non-AC defenses potentially improve by 15 (level) 6 (enhancement) 2 feat and 5 (attribute) for a total increase of 28 over 29 levels. There are a few random items in Adventurer's Vault that could add to that to make the chance to be hit at level 30 equal to the chance at level 1. Practically, however, that is not the way it is likely to work out because:
A. Characters will generally advance only two attributes by 10 points, non-demigod characters only advance by 8 points, and some characters will advance two stats linked to the same defense. (For instance, strength and con is a popular combination for some fighters and melee warlocks and wisdom and charisma are popular combinations for charisma paladins and laser clerics). Thus, at least one and possibly two defenses will only increase by a maximum of 24 over 29 levels, yielding a 50% increase in the base chance to be kicked in one or two NADs.
B. Most characters will not take all three NAD improving feats. (And feats should not be necessary in order to make the system work in any event).
The system is also balanced on the assumption (per the DMG) that NADs are usually 2 lower than AC. Practically, this does not usually work out.
For instance, a typical elf ranger in hide armor with an Str 10, Dex 18, Con 14, Int 8, Wis 17, Cha 11. AC 17, Fort 13, Ref 15, Will 13. The ranger's AC is within two points of his highest NAD, but is four points higher than both other NADs. Thus, if attacks are balanced with a 50% hit chance on the assumption that the non-AC defense is 2 points lower than armor class, the actually hit 60% of the time instead of 50% of the time--at first level. (And remember that this disparity will grow larger over time because AC will generally keep pace with monster attack bonuses, but only strong NADs will do so, and even then, only if you improve them with feats).
The ranger is actually fairly typical of characters and NADs. For instance:
Dwarf laser cleric. Str 10, Dex 13, Con 16, Int 8, Wis 18, Cha 14
AC 16 (chain mail), Fort 13, Ref 11, Will 16. Again, the strong NAD is equal to AC (though this is a low-AC character). The weak NADs, on the other hand, are 3 off and 5 off of AC.
Dragonborn strength paladin. Str 18, Dex 10, Con 13, Int 8, Wis 16, Cha 12
AC 20 (plate and shield), Fort 15, Ref 13, Will 14. NADs are between 5 and 7 below AC.
Half-elf control oriented warlock. Str 10, Dex 10, Con 18, Int 16, Wis 12, Cha 14. AC 15 (leather armor+Int), Fort 14, Ref 14, Will 13. NADs are between 1 and 2 of AC (and this is about as low-AC as characters come in 4th edition).
Human staff wizard. Str 10, Dex 10, Con 16, Int 18, Wis 12, Cha 12
AC: 17 (leather armor+staff of defense), Fort 14, Ref 15, Will 16. NADs are between 1 and 3 below AC.
Human sword and shield fighter. Str 18, Dex 14, Con 13, Int 10, Wis 14, Cha 8
AC 19 (scale and shield). Fort 17, Ref 15, Will 13. NADs are between 1 and 6below AC
Eladrin Brutal Scoundrel rogue: Str 16, Dex 18, Con 12, Int 12, Wis 8, Cha 12
AC 16 (leather and dex), Fort 13, Ref 16, Will 12. NADs are between 0 and 4 below AC (if the rogue uses a double sword, that's 1-5 below AC).
NADs start out more than the 2 points behind AC that the system assumes (except for the characters with the worst AC) and only fall further behind as the game progresses (especially for non-demigods or characters who choose to advance linked stats (such as Str/Con).
In practice, it is a bit worse than that. The monsters that are most likely to have NAD targeting attacks are elites and solos--boss monster types who get additional attack bonuses and are often higher level than the PCs anyway. So, unless you have the good fortune that the monster is trying to kick your strongest NAD, you will often be looking at a 70-80% hit rate from the monster trying to kick you in the NADs.