A 12th level fighter has generally run out of feats to increase his damage by that point.
A 12th level wizard has had them since 7th level and at 12th is only just beginning.
12th level characters are not 1st level characters, they are in a different world, damage increases have to be brought into proper focus.
For example, a 12th level evoker casts two maximized fireballs while hasted. He also can throw out a quickened magic missile in the same round with two feats. Technically empower does more damage on average than maximize, so this isn't even an optimized example, but 120 fire damage over 1256 sq feet plus around 17 more points of damage no save to a singed looking survivor is not something first level characters are capable of.
Say you are able to get only 6 people in that 1256 sq ft area. And say 2 of them are either lucky or have good reflex saves or both. Thats (120*4) + (60*2) + 17 points of damage in a round. Or 617 points of damage.
Let's say this 12th level fighter has improved critical and decisive strike, he's got +12/+7/+2 base attack, one extra attack from haste, +5 strength, a +3 weapon, and weapon focus and specialization as is required. And he's using a greatsword because it's a very good weapon.
So, +12+5+3= +20/+20/+15/+10 for 2d6+10 damage with a 17-20 crit range and x3 damage. 4 rolls isn't a very good chance to get 17 or better, but let's assume he is lucky as well and not only gets a 17, but gets it on his third attack so he's still got +15 to confirm it.
Against an AC 25 character he should probably power attack around 5 because his fourth attack is not likely to hit, and 5 power attack triples to 15 damage if he does land a crit. Actually, 15 is less than 2d6+10, so he should probably not power attack at all. Ok, so he doesn't power attack.
He rolls 7, 12, 18, 15 for 4 hits and one threat. He confirms the critical against a non-critical immune character (there's another balancing factor to the feat, the sheer number of creatures it's going to be useless against----especially at high levels where moderate fortification should be prevalent to stop improved crit falchions and 8d6 sneak attacks) and does (2d6+10*3) + (2d6+10*3) for on average 51+51 or 102 points of damage. 102 points of damage may drop his opponent, but then again it may not at this level. And 102 isn't even as much damage as the wizard did to 1 target, much less his total damage for the round.
That's hardly overpowering. Can it be broken? Yes. What can't? Is it broken to begin with? No. It's a balanced feat that allows fighters to maintain utility at high levels as opposed to degenerating to flesh walls for the real heroes the wizards.
The fighter's feats matter less and have to be judged on a weighted scale because typically his damage can only effect one target at a time.
It's just like the weighted scale for judging what level a spell should be. A 5th level spell has a 20 dice damage cap for single target spells, but only a 15 dice damage cap for area of effect spells because over an area, the wizard can do much more damage.
In any case, yes it's powerful, but with a minimum requirement of 12th level, 3 feats for prereq's, rule parallels to existing feats such as power critical and improved critical, it is well designed. And when taken in the context of a 12th level game, is just a drop in the bucket and a good but not always advisable (or even practical) choice.
For instance, your average paladin or ranger or barbarian might take weapon focus or improved critical but will they have the feats for both? With mounted combat/ride-by/spirited charge and or exotic weapons, or power attack/extra turning/divine might, or power attack/cleave/great cleave, and especially if the character isn't human, decisive strike becomes a nice trick, but too expensive for a 3 feat (3, 6, 9) character..
It's not a fighter only feat (that would make it even harder to acquire) but it is very close with 3 feats for requirements.
There are any number of reasons here why it is a balanced feat. If there is that much to say, surely the feat is in line with what it should be. Even if my reasoning is off in 2 or even 3 places, there's still 5 other reasons the feat is balanced.
Shift your mindset to high level gaming (which BAB 12+ dictates it must be) and everything comes into focus.