I would be against a post count badge because some people get obsessed with post counts on Internet forums and start posting endless crap to pad their numbers. Rep and Likes show that someone has quality over quantity with their posts, and that's the most important thing IMO. We've all seen members who show up and run up tons of posts in a short time and then disappear. For that reason, recognizing those who contribute over a long period like a year would mean more to me.
I think a useful badge would be one for those who have completed at least one grow log on the site. That not only shows that someone is willing to take the time to make a real contribution to the site, but also that they're speaking from real-world experience rather than just repeating what they've read elsewhere.