...I would create a mechanism that recognized OPs who started engaging threads.

While FT shows post counts and join dates, I think FT would be even better if it showed an indication of how interesting the user was to the rest of the FT community. The easiest way to measure a user's interestingness would be to count the number of replies to threads started by that user. Perhaps there could be a badge for posters who started threads that generated 100 replies, another badge for posters who started 1000-reply threads, another badge for 10,000 reply threads, etc? (The count would only include replies that met a certain minimum word count, so that "+1" and "Concession Accepted" replies would not count towards the total.)