MongoDB offers some REALLY cool features that enhance development speed. Inserting into a table that doesn't exist? It creates the table on the fly. Querying from a table that doesn't exist just returns an empty result set (instead of an error, or exception).
If I had to speak in specifics though, I think it's pretty easy to say that NoSQL stores allow for querying very large datasets faster than you could on a relational store on similar hardware.