Right now you can only shove a person down if your weight is heavy. This seems foolish. Many enemies can knock us over while clearly being less encumbered.
Lets take the idea and suggest that everyone can use the knockdown feature. How to make it useful? Light against light knockdown would stagger, like a punch to the face. Medium against light would knock down like being dropped to the knee. Heavy against light would bowl a person over like they do to you (think crestfallen knight with a shield charge). Heavy against heavy would be like light against light, and so on and so forth. And if you tried to use this against someone with a heavier set you would stagger. Medium against heavy just stagger, and perhaps light against heavy they could counter it. Each level of stagger or knock down would give you more or less time to deal with other mobs. When there are three v one it would be particularly useful to slap a guy down in medium, and to bowl over a light guy with a shield charge so that you could deal with the heavy one on one.
My point is that this mechanic has a lot of room for work, and to make heavy armor sets appealing without being in a lighter class every weight class should have access so that the rewards for heavier sets of encumbrance will be more greatly felt.