A light fighter is basically a fighter with better sustained damage than most bruisers, but not as much tankiness, who's supposed to use maneuverability or abilities to survive? I think, anyway.
I think zky is right that Fiora/Yi are basically just assassins. Fiora is actually in line for a rework specifically because she's too much of an assassin and not enough of a fencer.
I think of Riven as a light fighter because her effectiveness depends on using her Q mobility, her shield, and her stun to both damage her opponents and avoid their attacks. I guess Yasuo is along the same lines, except he gets even less survivability and even more damage.. I don't really think of them as assassins because they don't have resets or escapes -- they're meant to hang around and win fights rather than go in hard and then leave fast.
I agree that light fighters can't be bursty or they just become assassins.