I loved the masks of power concept and the idea of changing powers with different masks, the elemental stuff was another cool thing, sort of like avatar before avatar. What got me was also the concept of biomechanical beings, half robot animals roaming an island? That's cool.
Not only are they biomechanical, but they're tribal, biomechanical beings with no advanced technologies, always seemed cool to me to have tribal robots but not done in a way that seemed to be technologically advanced.
With the sets, it was just constraction, collectability and gear functions, in the later years it was because stuff got so much more awesome, giant vehicles, jet bikes, combining functions.