Main point: Retains some friendly properties of SQL. (Query, index)
Key Features Of Mongo DB:
     Master/slave replication (auto failover with replica sets)
    Sharding built-in Queries are javascript expressions
    Run arbitrary javascript functions server-side
    Better update-in-place than CouchDB
    Uses memory mapped files for data storage
    Performance over features
    Journaling (with –journal) is best turned on
    On 32bit systems, limited to ~2.5Gb
    Text search integrated
    GridFS to store big data + metadata (not actually an FS)
    Has geospatial indexing
    Data center aware
    
Best used: If you need dynamic queries. If you prefer to define indexes, not map/reduce functions. If you need good performance on a big DB. If you wanted CouchDB, but your data changes too much, filling up disks.
Examples: For most things that you would do with MySQL or PostgreSQL, but having predefined columns really holds you back.
Regards,
Alok