MongoDB is teh awesome because of a simple query syntax, the ability to shard data across machines easily, and the ability to store files in GridFS while taking advantage of replication & sharding.
Indeed, I think the combination of query syntax and GridFS makes MongoDB unique.
Sharding is supported by many other NoSQL databases and for many of these things are even simpler than having
mongos, etc. Between document databases, CouchDB has recently got BigCouch to address the scaling issue
As regards querying, one could say that having MapReduce around would get you similar functionality to MongoDB queries. But starting with users’ familiarity with using queries vs programmatic querying and up to execution behavior MongoDB queries and MapReduce are quite different.
Original title and link for this post: 3 Reasons to Use MongoDB (published on the NoSQL blog: myNoSQL)