I recently installed Matthias Pfefferle’s web mention plugin. It is a great plugin and hopefully when he has polished it up further he will push it out to the wordpress.org plugin repository so it can enjoy a wider audience. I actually think the Jetpack team should look at including an extended set of this functionality in its plugins as the potential for distributed sharing that the indieweb provides could be a major distinguishing feature for WordPress, and certainly a better bet than trying to roll their own social networking ecosystem.
However thinking about the architecture of of webmentions I think they could be extended by becoming bidirectional. At the moment of the method is built around a POSSE architecture. This works well for long form articles which can stand alone but address issues or ideas that are posted on an external website. But if that is all there is to webmentions it is just a nicer implementation of Pingback.
More importantly though webmentions vision includes distributed commenting and it does succeed at making this possible, and members of the community are making use of this functionality. However the current workflow is awkward and i doubt it will catch on with the general public. Sophisticated Indieweb users can and will read an article on an external site and then return to their own to post a comment, but that king of behaviour is not intuitive.
But all is no lost as the webmention protocol could easily be implemented to support PESOS, which when commenting is more much natural. This is best explained by illustrating a use case namely:
A commenter (for this example me) arrives at external site, reads an article and wishes to make a comment. I make the comment on the external site and as part of making the comment I add my author url, that being the url of my own site. The external site then sends a webmention of the comment to my site. My own site could then scrapes my comment and saves a copy in my CMS. Otionall I could republish the comment in my blog or activity feed at my discretion. Thus fully implementing PESOS.