Blogger Bryce Nyeggen understands exactly what MongoDB is — "it's just mmap":
Basically, MongoDB consists of a collection of mmap’d linked lists of BSON documents, with dead simple B-tree indexing, and basic journaling as the storage durability mechanism (issues with what the driver considers a “durable write”, before data necessarily hits the storage layer, is something others have dealt with in depth). Eventually writes get fsync’d to disk by the OS, and reads result in the page with the data being loaded into memory by the OS.
– Bryce Nyeggen, "The Genius and Folly of MongoDB"
That's right, Nyeggen makes it pretty clear that MongoDB doesn't really bring much new stuff to the table. Whereas Cassandra has an innovative consistency protocol, Redis has "crazy data structures," and Hadoop has powerful batch data-processing abilities, MongoDB has mmap.
But that's okay, he says. Innovative databases can also come with a lot of learning and development overhead. See what he means in the full article.