Here's just some thoughts I have on archivability.

1. Is this a post that can never truly be replicated, i.e. really valuable information, an epic poster fight, or just some really funny reply? If so, it should probably be archived.

2. Is there valuable information/discussion in this post that truly captures a moment of good in-depth analysis/conversation in PTP?

3. Is this post screamingly funny? Does it have a classic beef between posters that people will talk about for years?

Those are my big three requirements. Add on?