I recall communities that would have a group for 500 posts, 1000 posts, 2000+ posts. I've also seen this as "level 1" level 2, 3... etc. I think it is somewhat cool to recognize users for being a long time member and while not all members are going to get premium memberships, it is cool to watch their growth in the community with a differing group. Perhaps they don't have any incentives other than the group, but what are your thoughts?