I seem to talk about this a lot - on my blog and with a couple of training partners.
Forgetting the current, modern-day boundaries - the SE Asia land mass has no really defining boundaries (other than the Mekong River). So people are free to wander from one place to another, observe and add to their life what they choose. Add in the wars of conquest that went on (looking to the similarities of Thai and Khmer cultures) and you have a lot of good ideas crossing the region.
Plus, having the vast coastal area for the trading and exchange of ideas that that brings, will add to the knowledge exchange quite a bit.
To me, most of the differences begin with what part of the martial arts is emphasized, and what part is not trained as much.
Thanks for letting me bloviate :supcool:
Don