So I did some research last night (about two weeks ago at this point) and it seems to me that Unarmored Combatant is a little bit over-powered. Basically, what I was trying to do with it was to simulate the Monk unarmored bonus to AC to account for the advent of firearms and the fairly impractical nature of putting are leather armor to help stop bullets from hitting you. I figured this wasn't a big deal because the 3.5 book Tome of Battle has a feat called Superior Unarmed Strike that gives a non-Monk a Monk style unarmed attack that improves over time. I juiced it up a little so you weren't stuck dealing 1d4 damage with your fists for three levels of play, but you still don't get to do 2d10 damage with your fists at high level like a Monk does.
Now with Unarmored Combatant, there are three tiers to the feat, each improving its armor bonus. This culminates in a +10 armor bonus at 20th level on the third tier feat. The problem is that a Monk ends up with +5 at 20th level, and to have the equivalent bonus, he'd have to have a 20 in Wisdom as well. Not completely outside the realm of possibility, but it is still a robust score. The point with these feats is to be able to do something your class normally can't do, but not be better at it than the class that is an expert at it.
So I think what I'll do eventually is alter the series and bring it to just two feats instead of three. The first feat will still require Dodge as a prerequisite along with an above average Wisdom or Dexterity. All it will do is add a +1 Monk style bonus to your AC or your Wisdom modifier, whichever is higher. Like the Superior Unarmed Strike feat, if you are a Monk, it will treat your Monk level as four levels higher when calculating your AC bonus. The second feat will have similar requirements to the second feat in the series, but will offer a unarmored AC bonus that increases with your level starting at +1 and capping at +4. This makes it good, but not better than the Monk's ability.
Now, that isn't especially great, at least not compared to the +10 at 20th level of the third tier of the feat series as it stands now. However, it does make a lot more sense when compared with the mechanics of the Monk ability and the Superior Unarmed Strike. If you take it at earlier levels, even if you have the bare minimum of stats to take it (Dodge feat and a 13 in Dexterity) it still comes out to a +3 bonus to AC, which is the equivalent of studded leather armor without the armor check penalty, the spell failure percentage, or the weakness of being ignored by guns. However, this does present a problem at higher levels. With only the base requirements it ends up as a +6 bonus to armor, which is the equivalent of chain mail or a breastplate without the armor check penalty, spell failure percentage, or the weakness to firearms. But it can't be improved by the upgrade system or magical bonuses.
It isn't quite so over-powered, but it is much more in keeping with the power level of the Monk ability. With a Dexterity of 16 or something along those lines you can easily scoot the bonus up to +9, which puts you right up there with full plate with none of the weaknesses of the armor.
I think in future version of the campaign book I will alter the way the feat works, but I won't change it now, as my players have designed their characters around it working in a certain way.