I'm baffled as to why people are still making guesses at the "D" part. It's obviously dual motors. That's it. Nothing else.
Also, I don't think the "something else" is Driver Assist. Those are features found on dozens of other high-end (and not so high-end) cars. Announcing that at a big event is like admitting that Tesla is playing "catch up" with all the other car makers who have had lane change warnings, and ACC for YEARS!. Elon wouldn't make a huge announcement over copying what everyone else has been doing for quite some time.
It's also not Automated Driving. Do you guys have any idea how far away we are from that? And not from a technological point of view, but from a regulatory, legislative, and insurance standpoint? Elon already said it's a few years away (I thin he said 3-5 years). He can't even get the Model X rear-view cameras approved yet.. you think Automated Driving is just around the corner?
Finally, if I were Tesla Motors making an AWD Model S, I'd put a much smaller motor to drive the front wheels in situations where it's needed (snow, ice, etc) and have it switchable by the user. In a Model S, one only really needs AWD for better handling in those environments, and a small, torquier motor would do the trick, AND consume much less energy. The idea of a dual-twin motor in the MS pushes it into supercar territory, and while there is a small market for that, I think TM wants to continue to being electric cars to the masses, and they don't do that by making a $200,000 dual-twin AWD motor supercar. They do it by adding a small second motor (driven off the single inverter) and keeping costs and prices as low as possible.
Now for the Model X, that might have dual-twin motors because of the increased weight and for marketing purposes. I'm not even going to guess on that.
I'd guess that the "something else" is slightly higher range (slightly bigger battery, and drop the S60), or maybe the first reveal of what the Model 3 will look like, like they did for the Model X two years ago. Or a better seat retrofit program for the Model S.