I’ll bite.
I don’t have TKD experience, but my lil’ bro does and I used to watch him and we used to mess around. I also have sparred kick boxers with a TKD background.
The first and most obvious difference is that the thai roundhouse is an unchambered kick. It swings directly up from the ground as though it were a “deadleg.” The power is generated by hip torque, not a snapping motion. The leg is very relaxed on impact. You are trying to bury your shin in and through your opponent. The extreme amount of power generated by kicking this way precludes striking surfaces other than the shin. Attempting to use the foot or the ball of the foot, would probably break something.
A TKD’er tends to chamber their kicks. They raise the leg up first, and then snap the kick out there. IME, this does not create as much power as the thai kick. That doesn’t mean it won’t work, and the philosophy is different as well…
Now, while all kicks are legal in thaiboxing, the push kick and the roundkick are the two most common. The push kick is a chambered kick, and while it is thrown quickly, the purpose is to maintain distance/set your opponent up. It’s not usually thrown as a “damage” weapon. Think of it as a boxer’s jab. Used for distance, probing, and set-ups.
A TKD’ers chambered kick has an advantage over the thai roundkick in that chambering the kick disguises the kick you’re going to do–a sidekick, a front snap kick, and a round kick are all initiated the same way from the outsiders point of view, and you can adjust what kick you are going to do fairly quickly if need be.
Multiple kicks can be thrown in much less time.
As bad as Thai Boxers hands tend to be, they have been better than the TKD’ers that I’ve met and played with. Thai boxers are intimately familiar with the clinch and fire knees and elbows in it. They also have an arsenal off off balancing “takedowns.” Not really throws, not really kicks, sort of a footsweep thing.
Thaiboxers are usually more experienced giving and receiving leg shots. They also usually have more conditioned legs that can take the heavy beating of throwing and blocking kicks.