The best mitigation for ranged damage is to keep moving, especially if the AI is stupid and just shoots for where the player is currently located. Anything that lets you fire while moving is cheap damage in terms of how much damage you take in response. This is clearly the whole point of Strafe, Evasive Fire and Vault:Action Shot. Another approach is to make use of occasional skills like Spike Trap, Sentry or Rain of Vengeance - they do a lot of damage relative to the time you have to spend standing still.
To be honest though, I think the main mitigation for most characters will come from gear and tactics, with defence skills used in emergencies. It could be the Wizard is better defensively than the DH, but that doesn't mean DHs can't survive. For the more defensive builds, a lot will depend on how easy it is to get Discipline (e.g. we don't know the 'chance' on Night Stalker).