Initially I didn't wanna make Water weak to poison because, well you can find a reason to make anything weak to poison, and because a Water pokemon is not made of water or even has to live in it, but merely specialized in Water based attacks.
So I kept the Poison reasoning to technique types...and noticed Dark, the only type besides Normal that doesn't have a real elemental ability or strength, it just plays unfair and uses dirty tricks. And faking a strong immune system wont make health issues go away.
But thinking about it again, and how everyone always first thinks of Water as something that should watch out for Poison attacks, maybe that'd make sense after all, and Water is defensively doing great anyways, so bringing it closer to Grass' "league" would be fine.
Grass taking neutral damage from Flying would be a good change, I think. While I understand that the reason behind it is that birds eat seeds and bore holes in trees, on the other hand wind doesn't kill plants (although strong winds can fell trees). In fact, plants need air to survive, and even create air of their own, so their relationship should make it neutral. As for Steel, I don't think Electric needs to be strong against it; while metal conducts electricity, electricity also powers machinery. Also, Electric doesn't need the weakness to Grass, because you can't kill a lightning bolt with a tree; if anything, the tree will start on fire, so the neutrality to grass attacks makes sense to me. =P That, and Ground attacks are still common enough that Electric-types need to be wary of them. I think Electric is pretty balanced as is, with strength against two types, resistance to three, resistance against three, and is completely stopped by Ground-types.
Giving Ice resistance to Ground is genius, though. Only two types can resist Ground attacks, but both of them are uncommon types. This is a great way of making Ice Pokemon better defensively. Makes sense in reality, too, as the ground gets covered by ice.
Also, Rock does need resistance to Rock; dunno why they haven't done that.
But which type should specialize in instability? Fire has burn, Ice has freeze, Electric has paralysis, Poison... well, that's just obvious, Normal (sort of) has sleep, Psychic and Ghost have confusion... good question, indeed.