More indexes per collection
Faster index creation
Configurable fsync time
Several small features and fixes
There are some changes that will require doing an upgrade if your previous version is <= 1.0.x. If you're already using a version >= 1.1.x then these changes aren't required. There are 2 ways to do it:
use a slave
start a slave on a different port and data directory
when its synced, shut down the master, and start the new slave on the regular port.
Ask in the forums or IRC for more help.
There have been minor changes in replication. If you are upgrading a master/slave setup from <= 1.1.2 you have to update the slave first.
mongoimportjsonhas been removed and is replaced with
mongoimportthat can do json/csv/tsv
We've changed the semantics of the field filter a little bit. Previously only objects with those fields would be returned. Now the field filter only changes the output, not which objects are returned. If you need that behavior, you can use $exists