It is easy for relational database folks to dismiss MongoDB as "silly". Research veterans have poked fun at it, there is a great video on youtube ("MongoDB is Web Scale") that cracked me up with its criticism of Mongo fan-boys, a recent VLDB paper from Wisconsin compares MongoDB and Sharded SQLServer on the YCSB benchmark showing that Mongo's performance falls way behind.
I think all of this misses the point. IMHO, MongoDB is not trying to be a more scalable, higher performance database for application developers who couldn't get this from sharded MySQL or any of the other commercial offerings. To understand why Mongo is so interesting, you have to look at the entire application programming stack, and how it is different from the previous generation. The standard 3-tier architecture of database, app/web-servers, browsers is getting rebuilt using very different technologies driven by the enormous amounts of effort pouring into building applications/sites for mobile devices: