These are great. I like @siggy47's method a lot -- generally, I love when people find ways to use what's there in creative ways, and he did.

Other than that, I like all your ideas, esp #5. Being able to fluidly talk about and refer to other things is so key -- I think of it as capital formation: building new, more complex, and more valuable goods out of simpler goods.

I think perhaps a lower-hanging fruit that might support a number of these things is a kind of 'scrapbook' feature, where you can save little pieces of things: posts, comments, your own thoughts as pertain to those things. The existing notifications tab is kind of a poor man's version of this, for me -- I remember things that I've said or replied to, and they're in notifications, so that's how I find them again when I want to talk about them. Seems relatively easy (compared to the others) to do a half-baked version of that.

I guess I could just do that manually, with a text file or something? Maybe I'll try that and see how it works. Although I guess that's not a very good solution for the average user.