Maybe it's because I've never understood the whole "achievement" thing anyway, but I can't come up with a reason to care about them at all. I can't think of a reason to use a system that restricts access only to people with completely arbitrary credentials. I can't think of a reason to implement such a restriction, either.
I can think of a reason NOT to export my application's "badges" to other applications, if any: it would encourage people to create accounts on my system for the sole purpose of gaining these badges. It further encourages behavior designed to obtain badges.
That said, I don't want to be too down on the idea. It's got some potential as an information hub. I would consider losing the "badge"/"achievement" terminology, downplay the "access control" angle, and bill it as a more generic hub for sharing information about what users get done in a given application.
I could see benefit of sharing information about -- say -- how often a user makes posts on user-moderated discussions, as opposed to voting, or that sort of thing. Swapping numbers and ratios instead of boolean data reduces the arbitrariness of the thing. The ability to use them in subtle ways, as opposed to ham-fisted restrictions, would likewise reduce the temptation to create throw-away accounts.
I doubt anyone would make a (non-spam) system that rewards people just for signing up. The site creator would be careful to reward only meaningful actions. So this isn't realistic.
>> It further encourages behavior designed to obtain badges.
That's the whole point of the badge itself though - encourage a particular type of behavior. I don't see this as a problem either.
>> bill it as a more generic hub for sharing information about what users get done in a given application
Very interesting take on the idea.
For example, HN could have a "1000 Karma" badge, which would be meant to encourage thoughtful, intelligent submissions and posts. People could instead say majority-pleasing things. Even worse, people could collaborate to vote each other's posts up. Get 32 such people together, have each make 32 posts in a topic that left the "New" page without getting any votes, and presto! 1024 Karma for All!
There are many variations on these themes. Badges (in social/multi-user settings) largely fail at getting people to do what you want, unless what you want is for people to waste time getting badges.
Now, in a closed system, this can be minimized: people who would normally read and post on HN are less likely to resort to these shenanagins, and others just wouldn't care. The aggregator gives them a reason to care.