I implemented webmentions on my site. They're like a modern, open implementation of pingbacks, a way for people on the web to be notified when others link to, "like" or "repost" their things.
Right now it's totally a DIY thing so hardly anyone uses it. At some point, plugins will be developed for WordPress and other CMSs and it could really take off at that point.
I see only one real problem at this point, and it's a big one that could easily sink the whole thing: webmentions are totally ripe for abuse.