Another big difference between the two styles is the level of conditioning expected, at high levels, amazing conditioning is expected from all martial artists, however, for Muay-Thai, there is a huge emphasis on muscle endurance and cardiovascular endurance straight from the get go. Daily 3 mile runs, bodyweight circuits and endless hours of pad and bag work are expected from the beginning, this leads to quite a steep learning curve that is quite intimidating to newcomers.
Whereas with most kickboxing styles like American or Japanese, there is less of a demand for high level conditioning so early on, which leads to much more accessibility to people and much much less intimidation. you could make an argument for either side, personally I don’t really know where I stand, I think conditioning is a hugely important aspect of martial arts but I also think it shouldn’t be introduced as quickly or as aggressively. Its similar to my thoughts on sparring, very important to do, but the approach should be measured. Conditioning is vital to success in both martial arts, you would be hard pressed to find an effective martial art where conditioning isn’t important, and if it isn’t, then that martial art is probably a load of shit.