4th edition D&D follows a very nice idea of trying to enforce a fundamental game balance. One example is that attack bonuses and defenses go up greatly with levels, but instead of letting them diverge greatly (resulting in fights where some hit rolls are impossible and some almost automatic), they specify exactly how quickly they should tend to go up so that difference doesn’t become too great. So the chance of a 30th level character hitting a 30th level monster should be similar to that of a 1st level character hitting a 1st level monster.
They specify that monster attacks and defenses are supposed to go up by one point per level. For characters, you can calculate how quickly they will go up, based on level, statistic bonuses, and magic weapons. The numbers work out nicely if you calculate the attack bonus a character will get over 40 levels (of course, in reality there aren’t that many levels). Characters will get +4 to every statistic and +10 to two statistics of their choice, so a total of +14 to their primary statistic. They get +1 better magic weapons every 5 levels, and +1 to hit every 2 levels automatically. This means the bonus is:
+20 from levels, +8 from magic, +7 from statistics = +35
So characters gain +7/8 attack bonus every level, slightly slower than monsters. Presumably the characters are supposed to make up for this in all the little bonuses they get from such things as paragon paths, feats, epic destinies, magic items, and so on.
Now, a character’s defense in light armor will go up at exactly the same rate. But because heavy armor is not based on a primary statistic, it goes up much more slowly. To compensate for this, D&D created the idea of masterwork armor, which gives larger bonuses for high level magic heavy armor. In theory, masterwork light armor is not really needed for game balance, but it would seem funny not to have it, and it may help compensate for high-level characters getting fewer ways to boost defense than to boost offense.
One thing that greatly troubles me, however, is the omission of “masterwork amulets”, extra bonuses for Fortitude, Reflex, and Will defenses. These defenses naturally go up more slowly than light armor bonuses. To be more specific, your special defenses that correspond to the statistics you raise with levels should go up at the same rate as your light armor AC, but your other special defense(s) will go up much more slowly. So without masterwork bonuses, special defenses would go up more quickly than heavy armor AC but more slowly than light armor AC. Therefore, they should get masterwork bonuses in between those two armor types.
The fact that there is no masterwork bonus means that special defenses go up more slowly than armor class. Since monster attacks that go against special defenses go up at the same rate as those that attack armor class, the relative accuracy differential of such attacks goes up. Worse, this does not obey the idea of the attack/defense balance remaining the same with levels. A 1st level brutal rogue, for instance, might be fairly easy for a 1st level monster to hit with a will-based attack. But a will-based attack from a 30th level monster will hit a 30th level brutal rogue just about automatically. This is rather annoying, especially if the monster has something like an immobilizing attack, and you know there is no hope the monster will miss you.