3.0: Elemental Weakness/Resistance?

[Yukiko]

This will result in people excluding certain classes and will make the Duty Finder a big risk factor when it comes to clearing content.

Thats already happening, have you never heard of "BLM burn strategy" or leavers in DF just after joining?

Almost no damage? Playing with fear to argument is a bad way! You do not know how they may implement it, there are way too many possibilities!
One way as example is to give the player resistances and not the boss, at least the players have more choises then and can prepare for bossfights.
Other way is to not make bosses immune to elements but even then there is not a single job or class with just "one" element, seriously does it really hurt that much "not" to use "one" element in your rotation?

Edit: Furthermore, it allows new classes/jobs with supporting abilities who can negate elemtal resistance as we seen in older FF games.