What are the most nuanced or 'hidden' KPIs in Community Product Marketing that aren't immediately obvious but are critical for long-term success?

I mentioned this in another answer, but I think a big one is community stability. At Reddit, our platform is entirely dependent on our community. In order for our users to have a good experience with every visit to Reddit, they need to trust that the community is secure and stable. In fact, they need to trust it so much that it never crosses their minds.

If a product rollout has an adverse impact on the community, it doesn't just affect the success of that product launch; it can undermine the integrity of the entire community infrastructure. I always want my team to consider community stability as the bedrock for any of their go-to-market plans.