Everything. Different tip shape, different sidecut, different core profile, different glass. The only things that are the same are the sintered base, sidewall material (though the rocker light is vert walls), edges, core material and that they're both handmade in Huntington beach by someone special.
The Rocker Light is stiffer and maintains its super jibability with the rocker, the Park Flat is softer but the tip doesn't lag when you snap it.
**by lag I mean nothing more than standard rocker pop delay. Skate pop is the term I guess.