Ever since this topic came up, I've sort of thought about how it could be implemented. Personally, I feel that the only way this would work would be to structure it in such a way that there is very little incentive for cheating. As I think was mentioned in this thread,
http://backloggery.com/ is a website that keeps track of which games you own, and which of them you've beaten / completed. The whole thing is on the honor system, and since there's no real type of leaderboard, there's not much reason to lie. Even if you do, people typically won't care / even notice you.
I see this idea implemented as a nicely designed website, handled the following way:
• There's an official list of games, provided by the maintainers of the site. Requests for additional / missing games can be requested and reviewed before being added.
• Achievement lists have a max number of achievements, say 50. Perhaps games will start out with a 'vanilla' list of achievements, but any additional achievements are user submitted. Users can up or down-vote each achievement, so that each list will eventually reflect the community's current ideas on what constitutes the 'best' list. Achievements that get bumped off the bottom of the list go into some sort of repository and can potentially get upvoted enough to get back on the list.
• Saying you earned an achievement is on the honor system. You'd probably have a progress bar for each game you start, and whenever you check off an Achievement it fills up a little. You can also switch to detailed view. People serious about this would probably live stream their games like they do on backloggery.
• As I mentioned, no leaderboard, and no way to search for something like, "who has the most achievements / highest # of completed games," etc.
• Able to add friends and comments and stuff like Backloggery.
Anyway, if I had the skill, time, resources, and wherewithal to pull off a website like this, I think I would. It
seems like it might work, and I for one would be happy with it.