Well, it seems to me that the scoring system is completely biased toward assassins and bursty champs. This is because of kill score, and its ridiculous. Why do we value the one who got the kill so much more than the other player who's contribution was also necessary to get that kill?
Well, here is my suggestion; creating a singular number to represent the kda contribution within the overall personal scoring system. Assists need a greater degree of definition for one thing.
a kill = 1
a hard assist = .75 - 1 (stun, taunt, charm, fear, high damage)
a medium assist = .5 - .75 (slow, silence, snare, knock up, medium damage)
a minor assist = .25 - .5 (Aura's, buffs, shields, minor damage)
Now, there are other issues with the scoring system, however, this would go a long way to address this one.