Here are a few of the gems I came across when looking at the docs:
1. Lots of small, but significant changes to the Chatter objects: LastModifiedDate on the *Feed entities allowing you to sort on most recent FeedItem or Comment, CommentCount and LikeCount include on *FeedEntities (I can hear the cheers from here!), and plenty more.
2.Customizable error messages to display to users when their password is locked, or they are changing their password.
3. New ContentDocumentLink object allows you to access the document in Salesforce content and where it is shared. This is super helpful when uploading a file via Chatter, and knowing where it is shared etc. This leads into another important change, you can now query,edit, and delete chatter files using the standard ContentVersion and ContentDocument objects.
4. Development Mode Footer shows Controller Extensions – I saved my favorite gem for last. This minor change in the UI saves an amazing amount of time when coding online. Coding in the cloud is all about working smarter after all!
What gems did you uncover?