The news about replying to Facebook comments
directly from your email inbox is a warmly welcomed one. This feature will most definitely generate more social activity through Facebook's platform, which will in turn generate more crawlable, indexable and sellable public content. This intent is obvious because the new feature does not include inmails, even though the technology behind replying to inmails via email would be far less complicated then the one behind replying to comments via email.
By enabling email discussions, Facebook is de-constructing its own walled garden by taking away the link that was taking users on its site over and over again. On one hand, discussions will probably explode. On the other, Facebook might experience a slight loss of traffic velocity (because of email's remote activity). However, since data generation will increase, Facebook will become more competitive on Twitter's grounds, and we all know by now how dear this is to Facebook (and there's nothing wrong with that).
Hopefully, this feature will be enabled on Fan Page Comments as well. This will help all those Fan Page owners who are struggling to create interactions around their products.
As a side note, I wonder how much of the Friendfeed intelligence was involved in this new feature...