The biggest difference is that serger thread is finer; because it needs to go through a number of different guides, any roughness would quickly create problems, either snagging on those guides in short order or gumming up the sewing machine. Because of this the individual filaments are also longer than normal, being more evident in the difference between inferior and superior quality threads; a shorter filament is more likely to break away from the main thread than a longer one. Even the twist can create potential problems; an inferior thread has areas where it is thicker or thinner thanks to the twists in the thread, and these small differences can create issues of their own.
Sewfine