Am I blind or is the
community wiki checkbox missing ?
(I'm asking because I was trying to convert this question).
Questions can no longer be made community wiki by the author, you have to flag a moderator and ask for the questions to be made CW. Answers can still be made community wiki as usual.
We have provided some additional guidance at the blog:
Most of the time, you should be asking yourself “How can I improve this post so that community wiki isn’t needed?” Community wiki is like a cheese knife: it is a specialized tool to be used sparingly, and only in very specific circumstances.