Yes, it has likely been suggested before. And yes, it would be very hard to implement on a forum. But, similar to how some forums have karma, etc, achievement points or at least a score of sorts that takes into account post-count (or maybe volume overall, in terms of submission. i.e. word count); times quoted within a post, or having a topic you have started linked to or mentioned; popularity of a topic or poll according to reply count; and so on - all of these things could go towards getting various forum achievements and an overall community contribution score.
I'm thinking along the lines of how Wowhead gives bronze, silver and gold achievements for various things, from visiting the site daily to getting a comment liked/advanced. There could also be a change of colour to particular posts within a topic where users have liked / thumbs-upped / given a star rating to show their appreciation for something witty, informative or simply cool.