The other bloggers involved have argued that removing all outright immunities solves the problem. The design of Dust to Dust is a clear expression of my, and Stands-in-Fire's, position on immunities in games: we're agin' it. At the same time, I can't agree that this is the right tack to take, for a number of reasons. Part of the G+ discussion has been about whether it's reasonable for anything to possess immunity to fire, with some holding that even a fire elemental - that is, again, a creature formed entirely of flame - should be possible to burn, if the fire were hot enough, and others arguing that, if anything, dealing fire damage to a fire elemental ought to cause it to increase in size.
It's interesting to me that one of the commenters and Kainenchen chose the same analogy to argue against immunity: why should fire elementals have fire immunity if a creature made entirely of flesh is not immune to the blunt impact of flesh? I've had a hard time verbalizing why I think this analogy inappropriate, but touching one flame to another - such as holding two lit candles together - damages neither and creates a brighter flame. I think that the vast majority of creatures, and the entirety of spellcasting, should not have access to immunity, but I don't want to entirely close the door to fire immunity as a possible creature trait, for the sake of those poor fire elementals.
I also feel that changing immunities to very high resistance values doesn't address the core problem. Resist Fire 50 might as well be fire immunity, for all that a D&D fire mage can do anything about it. In 3.x, an empowered fireball averages 52.5 damage, and shaving 50 points off of that means that the fire mage can nickel-and-dime the creature to death while just about any other kind of spellcaster operates at full effectiveness. This is why I say that a fire mage doesn't have as much fun as a complete generalist in such fights.