Hiya!
So...some people seem to be arguing that "Disintegrate" is basically "Polymorph to Dust Form"? Is that what it boils down to? If so, that's...odd...
Anyway...I typically DM with a sort of "If/Then/Else" type of attitude. I start with one thing, then just work down the logic-list until I get something solid. In the Disintegrate of a Wildshaped Druid...
First: Did the Beam hit the druid? Yes? Ok...
Second: Did the damage reduce the Druid to 0hp or lower? Yes? Ok...
Third: Druid is Disintegrated. What does that mean?
"A disintegrated creature and everything it is wearing and carrying, except magic items, are reduced to a pile of fine gray dust. The creature can be restored to life only by means of a true resurrection or a wish spell". Ok...end of logic list.
Well, I get to "Third" and that pretty much definitively means the druid is dead. Why? Disintegrate says so. If at the Third point, the spell description
didn't say "The creature can be restored
to LIFE..." I would have moved on to a Fourth. Which would be the druid description for Wildshape. In that, I would move on down the list until I came to a logical conclusion. If that was the case, wildshape says "0 hp = return to normal form", then the druid would transform from a pile of dust into his normal shape. HOWEVER, we only get to the Third point because it definitively describes the target as "dead" (re: "...restored to life..." <-- meaning he's DEAD at this point).
If I get all the way through a logic-list, and there is still no definitive answer, then I just make a gut ruling that favours campaign continuity (yes, campaign...not PC's, not "player fun", not "DM fun"...what I think is best for the continuity and continuation of the campaign).
*shrug* No matter what I say, run it the way you want it to run in your game. In your game, YOU are the one making the decisions. Not me, not someone on this list, and no, not the "developer of the game". There are a lot of people on this list who, IMHO, have *much* more experience at RPG'ing (playing, DM'ing, creating, etc) than
most of the developers. I give them a lot more credit and weight than the developers...especially considering the developers are kinda beholden to The Company's bottom line. If they can say "Grey", rather than saying "Black" or "White" (even if Black or White is the most logical answer), then they
will say Grey...they offend less people that way and it doesn't look like they are taking sides; this means both Black and White sides are still neutral in terms of forking over their $$$. If they get ticked off because a 'developer' said "Black", and so they don't fork over as much or any $$$...then The Company isn't happy. So...a developer saying "yeah, stuff and 0hp and druids...bottom line, Grey" is best taken with a grain of who-the-F-cares?

That's one of the main reason's I pretty much ignore "errata" and "developer tweets". IMNSHO, of course!
^_^
Paul L. Ming