Thanks for all the responses! It may help to distinguish two types of speculation (cf. https://terrytao.wordpress.com/career-advice/theres-more-to-mathematics-than-rigour-and-proofs/): "pre-rigorous speculation", in which one asks "dumb" questions (cf. https://terrytao.wordpress.com/career-advice/ask-yourself-dumb-questions-and-answer-them/) before fully knowing the field, and "post-rigorous speculation", in which one shares informed insights and opinions from one's rigorous understanding of the field.
I think we should encourage both types, in appropriate venues of course (and separated from traditional "rigorous" work).
@tao - One "safe space" for airing less precisely formulated speculations seems to be blog articles. But blogging seems to have declined in popularity with young mathematicians.
Another is Twitter, though I've mainly seen category theorists speculating there, not so much other kinds of mathematicians.
I hope Mathstodon will become another place for informed speculation!
@tao If a young and/or unkonwn mathematician was to propose an amazing conjecture accounting for several mysterious things and predicting heretofore unnoticed phenomena, would we really frown at him?
Based on the experience of Beilinson when he stated his conjectures in the late 70's, I would think that on the contrary the community would react very positively.
@tao I remember at a department dinner during my maths PhD I suggested to a table of eminent mathematicians that it might be interesting to have a journal focused on speculative but informed mathematical ideas. Total silence around the table until one of them said "you mean like a biology journal?". Guffaws all round.
I'm now a neuroscientist. 😉
@tao From the same thread ...
https://mathoverflow.net/questions/38639/thinking-and-explaining#comment90487_38639
@tao In a related story ...