Ask HN: Discussion of books/TV that prevents spoilers?

For those of us who watch shows long after their original airing, it often is a solitary experience.

Waching <i>Lost</i>, for example, as it aired allowed for fun speculation, but if you're getting into it now, it's prohibitively difficult to find analysis or discussion that is limited to what's known at a given point in the series.

It'd be great to have a discussion site with a slider to mark your progress in a given text/show and that auto-hides discussions tagged with info only known later.

I don't think there's enough there for a new site, but does anyone else think it would be a worthy feature for some forum to bolt on?