Seems wonky to me that they have a magic hitpoint shield created by their form, although there was always a healing component to changing forms in previous editions. I guess it depends on the range of hitpoints the different forms have.
Well it's not more wonky that the temporary HP from Rage or from spells.
I do think that one risk is allowing forms with too many HP, but according to the podcast the list of allowed animals will be fixed, so they should be able to avoid problems.
Whats the theory behind the damage taken in animal form not transferring to the natural form of the druid? I haven't looked at the DDN Druid and am only a little familiar with the 3/4e druids.
What's the theory behind the first damage taken by a raging barbarian burning up the temporary HP first, so that possibly he can be still at max HP when the rage ends?
There is no theory, just HP abstraction. Pick your favourite explanation...
The way I see it, the wildshaped Druid does not at all have 2hp when transformed into a mouse. She has all her normal (current) HP plus 2. That's what they said in the podcast, that you do not change your HP to those of the animal form, you gain temporary HP on top of yours. Taking 2 points of damage does not correspond to "killing the mouse" because it's not a normal mouse, and because HP are abstract and not only a measure of physical health. The current
suggested description of damage effects to HP even suggests you see no sign of injury until half your max HP and only "signs of wear, such as cuts and bruises" between half HP and 0.
Alternatively, since wildshape has in the past typically bestowed some healing, you can say that in this edition the healing takes place when you revert back.