Another difference that makes or breaks a system is realism vs heroism. While they work fine alone they don't mix well. d20 combat is a primarily heroistic system, while CT combat is primarily realistic.
That is not to say that realistic combat is like a finite element computer model of what happens when a bullet hits flesh. Rather, realistic combat says that a weapon hits "you", not some abstract buffer of points which is then decrimented.
The damage is applied to a Stat which represents a physical characteristic of your body. Now any attempt to use that physical characteristic, Strength for lifting a wounded comrade into the Air/Raft for example, is impaired.
In its raw form, the d20 character is like a potato. No bones, nerves, tendons, or muscles that have to function. Whittle away at it, a potato it remains, until finally the whole potato is cut into fries. Sure, you can add critical hits that define damage, a broken and useless arm for example.
For hand weapon combat among armored opponents, the potato model actually works fairly well. Penetrating the armor robs the blow of energy, and a more experienced combatant may be able to roll with the impact to further reduce its effect.
Firearms are another matter entirely. You can't dodge, block, the bullet move faster than you can see or react. You can't roll with the blow, the bullet imparts its large kinetic energy to a very small area almost instantly.
Another difference is combat time. My experience with d20 is limited to reading the rules and I can't remember whether d20 keeps the old (A)D&D combat rounds of 1 minute. The idea was(is) that combatants attack and parry, feint and maneuver, until one or the other sees a prime opportunity to strike. Again, this works well with armored potatos heroically crossing swords for the camera.
Realism, on the other hand, is the fact that I can fire shots from my gun as fast as I can pull the trigger. At 25 yards a good portion of them will hit a torso-sized target. A crack shot can switch between targets in a moment, hitting several targets in as many seconds. There will be some ducking for cover and shifting positions, but the action will tend to be over shortly or reach a stalemate leading to an interval of tactical maneuvering.
I've seen some d20 rules for martial arts, and they superimpose a blow-by-blow resolution on top of the generalized combat round. But then any other kind of combat should have feats the character can call upon to interrupt the game mechanics with special attacks. And apparently these do exist: bull rushing, disarming, tripping, maybe even the Black Knight suddenly hurling his sword through the eyeslit of his opponent's helm.
This begs the question of what happens with general combat hits or misses. If I hit, a thrust with my longsword it will do a different kind of damage than a slash or a chop. A blow that landed but failed to penetrate armor is different from a swing that was dodged or parried.
Why is the whole combat round not choreographed with actions, feats and maneuvers? Because the combat system isn't designed for it.
