New command for collection-level compaction, Better concurrency, Index Enhancements, Authentication with sharded clusters, Replica set priorities (set preference for which server should be primary), Data-center awareness, Polygon searches, ..a few other things...it's all in the release notes.

Yeah, but are there any performance improvements? I'll be waiting to see the blog posts about it that are undoubtedly forthcoming.

I can tell you I'm excited about the new indexes...this weekend I'm going to restore our production database into a local 2.0 instance and see what type of memory savings, if any, we get (since I'm cheap and ram is my biggest issue) (it won't be as easy for me to measure the possible improvement in performance mentioned).

"Indexes are often 25% smaller and 25% faster (depends on the use case)"

Huge performance increases, actually. The write lock yielding whenever the DB is hitting the disk is huge and is going to resolve a large number of my production complaints with it. The improved indexes are also quite tantalizing.