I think it would be a healing ritual for the MO community for someone of appropriate stature (which excludes me) posting an inviting cwiki question that elicits responses from the users to this significant passing, the equivalent to mathematics of Bill Thurston's untimely death. The (useful) notice, Grothendieck -sad news, doesn't quite achieve it. Perhaps something along the lines of Dave Roberts' query in the comments...?
The question at MathOverflow has now been closed; I have added a brief explanation as to why. For now it will neither be deleted nor locked, so that comments can still be added by members of our community.
I propose that a soft question be asked not specifically with regard to the passing of Grothendieck, but with the view of eliciting thoughtful reflections on the impact of his work. The present 'answer' is community wiki to encourage others to have a hand at formulating such a question.