I've followed Postsecret for quite some time now; every Sunday, I check in to see what secrets others have posted. The idea, as people quickly figure, is that the postcards show so many facets of just being human--having fears, frustrations, joys, and many a quirk. It only updates once a week, though, and there are only so many secrets.
LJ Secret, though, takes up some of the slack. The mature concepts warning is largely a precaution implemented to avoid lawsuits, I guess, from parents who don't supervise their children's online activity and get upset at what kids can find. The secrets here tend to be more computer generated imagery, but the concept still stands--people confessing secrets in absolute anonymity. Livejournal has a reputation for being a haven of drama and teen blogs, and sections of it can be, but it's got a varied, thriving community of different people, so the secrets display a similar range of identifiable sentiments.
It may be voyeurism, but I find others' secrets fascinating. I would like one day to work up the nerve to submit one of my own, but it feels so personal all the same. *shudder* No, not there just yet.