• Replication and sharding enhancements (since v.3.2)
• Document validation (since v.3.2)
• Aggregation framework enhanced operations (since v.3.2)
• Multiple storage engines (since v.3.2, only in Enterprise Edition)

MongoDB evolution diagram

As one can observe, version 1 was pretty basic, whereas version 2 introduced most of the features present in the current version such as sharding, usable and special indexes, geospatial features, and memory and concurrency improvements.

On the way from version 2 to version 3, the aggregation framework was introduced, mainly as a supplement to the ageing (and never up to par with dedicated frameworks like Hadoop) MapReduce framework. Then, adding text search and slowly but surely improving performance, stability, and security to adapt to the increasing enterprise load of customers using MongoDB.

With WiredTiger's introduction in version 3, locking became much less of an issue for MongoDB as it was brought down from process (global lock) to document level, almost the most granular level possible.

At its current state, MongoDB is a database that can handle loads ranging from startup MVPs and POCs to enterprise applications with hundreds of servers.