Happy to hear that MongoDB 2.2 has been finally released. Looking forward to use the aggregation framework & co.
Some Highlights from MongoDB 2.2 (see release notes)
The aggregation framework makes it possible to do aggregation operations without needing to use map-reduce. The aggregate command exposes the aggregation framework, and the db.collection.aggregate() helper in the mongo shell provides an interface to these operations. Consider the following resources for background on the aggregation framework and its use:
TTL collections remove expired data from a collection, using a special index and a background thread that deletes expired documents every minute. These collections are useful as an alternative to capped collections in some cases, such as for data warehousing and caching cases, including: machine generated event data, logs, and session information that needs to persist in a database for only a limited period of time.