So, if someone asked you why they should use "Riak and countless others" and not MongoDB, what would you really say? Also, you seemed to imply that Riak was a go-to solution (my wording) while implying that MongoDB was more of a fringe "buzzword" technology ... when, counting features, I think most would acknowledge MongoDB as being more mainstream.

There are a large number of well-established and quality installations of MongoDB. It works really well at both small and large scale and with a bit of tweaking (like any technology), can perform nicely.