Thinking more about what seems a little too powerful ( = steel & dragon), rather than what needs help.
Dark super effective on Dragon:
Because every story with powerful mythical beings has that one that was corrupted by evil. As for the knight comparison, it's not the steel or the strength that kills the dragon, but the cunning and the weakness-abusing blow that lets the puny man overcome the great beast. 'The harder they fall' and all that.
Dragon 0.5x instead of 1.0x on Ice:
It's a no-brainer, ice needs resistances, and it takes steel down a notch since it'd no longer be the only thing to effectively stop an outrage.
Electric super effective on Steel:
As explained earlier. And to counter the 'directing the current away from vitals' -argument: water has an electricity weakness based on conductivity as well, so the same should go for steel.
Psychic 1.0x instead of 0.5x on Steel:
Because being hard on the outside has nothing to do with mental defense, and if it's one of those pokes that does have an aptitude in that (Metagross, Bronzong) then it's already getting a resistance from the psychic half anyway.
And one more to go along with something that has been said earlier:
Water super effective on Electric:
Because it shorts it out. It's also not game-breaking, because it takes a lot of balls to counter fast electric pokes with water, due to the weakness working both ways. Well, unless your name happens to be Swampert, I guess.