To be quite frank, I believe that this idea would work well if every achievement provides some type of award that will motivate the member to gain. However, the issue with this is that eventually, the administration team will be swamped out with so much achievements, monitoring them, and validating the posts to see if they are actually working for them. For example; the first post you made was to get 50 posts and get free VIP, or something along those lines. Well, one could simply bump fifty times, and gain the rank in no time compared to someone who is taking their time in typing a paragraph. It's more so a quality over quantity type of ordeal
If the administration team can come up with some sort of script that automatically monitors, and distributes the achievements, I believe you have created a form of artificial intelligence, and ontop of that, something that will entertain the members within the forums. In addition, you will also bring in more members if this is actually created.
Sorry, I tend to ramble.
TL;DR
Better in theory than in practice.
Admins will need to monitor, and validate if a member actually completed the achievements.
Easily exploitable.
I Like to have my free name 
