The Muay Thai fighter trains with 60%+ power behind his punches and kicks. He wears gloves and so he can punch to his opponents head without worrying about breaking his hands or his training partners nose. They kick hard and never pull them. When they get close, they clinch and throw knees to eachothers body. They keep clinching until they throw the other to the floor.
This is one of the problems with a boxer or UFC or whatever type of fighter from any sanctioned competition style in a self defense situation. If you don't know how to punch without gloves and have not conditioned your hands and wrists and practiced punching bare nuckled you may hurt yourself more than the other guy with the first punch. And if you don't avoid hitting the other guy in the mouth you may end up with a bloody, highly infected hand as well. And once you put on gloves you remove a lot of options: grabbing, poking, penetrating strikes verses blunt trauma strikes, etc.
On the relative ineffectiveness of kicks that use the instep as a stricking point, I have seen more than one knockout from a roundhouse kick to the side of the head as well as some pretty badly bruised ribs, arms and thighs, however any trained martial artist would almost always strike with the ball of the foot, which is a very good penetrating striking surface which can break ribs and effect knockouts with ease.
One last comment. Depending on what school/teacher you study an art at, you can get a very different instruction. I know it is not TKD, but it is another "traditional" MA: I studied Shoryn Ryu Karate for a few years and my teacher and his school had only a dozen or so students at any time. His 2senior students were awesome fighters and even, although I would never advise anyone to do this they actually went to bars and got into fights and never once even got hit by anyone, before putting the other guy to sleep early. My teacher taught the traditional style, with a bent toward pratical self defense. At the peak of my study with him, like any other students who trained with him for a few years, I could take a full force hit with a baseball bat on my shins, forearms, ribs, thighs, etc., basically anywhere except the head, groin and throat without a second thought, like a well conditioned MT fighter with a few years of training would be able to, and I could easily break 1x4's with knuckles or the ball of my foot. Fortunately I never had to defend myself in a real situation, but I did spar and train with people who had, so I think I could have help my own with the average street thug, and I was probably one of his worst students.
Now, my teacher learned the art from the original master in Okinawa and studied there full time for a few years along with another American who happened to open a school in the next town. The other teacher's school had 100's of students and emphasized pushups, running and tournament fighting. I wouldn't have given you 2 cents for the ability of any of his students to be successful in a self defense situation. Two teachers/schools dervied from the same source with very different philosophies and very different results.
My point is, again, there are good teachers/schools and not so good ones in any style/art: choose carefully, or you will waste your time and money.
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