Stance and guard for starters. MMA is more square on whereas boxing is somewhat side on while MT isn't as side on as boxing. MMA a lot of people have their hands low, chest height, for takedown defense, in Muay Thai they're always up - same with most boxing styles. Muay Thai is taking on adaptations from western boxing in regards to footwork,head movement etc. Should probably add in as well that in boxing you'll see a bit of "shoulder roll" being used, especially by Floyd Mayweather. You won't get this in MMA and rarely in Muay Thai.
MMA striking is in my eyes very sloppy, a lot of wild hooks and haymakers being thrown in place of straight punching. Of course you get the ones that put time into striking and do well in MMA. Punching technique is almost universally the same, a jab's a jab, a cross a cross and a hooks a hook.