OH GOOD, MongoDB is a general replacement for an RBDMS... serious. fucking. facepalm.
It'd help if MongoDB were actually good or fast at something, or did something you couldn't do with a hashmap and text files, then I might be impressed.
Which market? To assume there is a single "database market" is to incorrectly lump document, key-value, graph, OLTP and analytic RDBMS into the same group. These are different tools for different purposes.