I think it would be a pretty interesting idea for the skill points that the player has invested in to have an affect on the players looks.

Lets say for instance, in Diablo 2, The sorceress puts points into mostly cold skill,s so the character takes on a blueish, more frozen look. The barbarian puts skills into stone skin, and his skin literally starts to look like stone.

I feel like this would be awesome, to see a character and have a general idea of their build based of how they look. This also gives depth into customization. You feel like you are actually developing your character, with the subtle changes to their appearance.

Witch doctor could turn slightly green from poison skills. Wizards hair stands on end a little and flickers with energy from his lightning spells.

To me this seems very cool. Has anything like this been asked about before (I'm sure it has), and has anything been said about it?

We do have skills that affect the way the player looks, as well as changes based on buffs, or if they're under the effect of a certain spell/poison/debuffs, what have you. Because of all of these things it's really important that these effects are very apparent, as well as easily distinguished, as they're important to gameplay.

It's obviously a really cool idea, but any visual changes to the look of the character could impair the ability for the player to quickly ascertain the visual cues being given to them. It's something we have to be extremely mindful of.