The comparison to the Pensieve is so so so good. I had previously used Tim Ferriss' metaphor for writing: that it was to freeze thoughts into a solid so that you could sculpt it into whatever shape you wanted. But as you've already noticed, there isn't really a final form a thought takes, and a lot of it's value is in bouncing amongst other thoughts.

I know this isn't a very hacker newsy comment, but wanted to highlight how amazing your comparison is :D