Lucid Imagination has an annotated version of the “Release Highlights” for Lucene/Solr 4.0

http://searchhub.org/dev/2012/10/12/apache-solr-and-lucene-4...

Spellchecking built into the index instead of needing an ancillary one is a big feature, IMHO.

Andrzej Białecki, Robert Muir, and Grant Ingersoll will be presenting a paper on the Lucene 4 architecture: http://opensearchlab.otago.ac.nz/paper_10.pdf

- near real-time indexing with soft commits

- zookeeper integration

Updating a single field in a document is a pretty big feature. It will require a much slimmer and less monolithic pipeline for big-data indexing from multiple sources.

Mike McCandless gives a good overview here: http://blog.mikemccandless.com/2012/07/lucene-400-alpha-at-l...

He has some other posts on there that do a deeper dive as well.

My favorite is the faster regex engine:

https://issues.apache.org/jira/browse/LUCENE-1606

The elasticsearch folks were waiting for that before exposing regexes to the world.

I agree with some of the other new and exciting features already posted.. but I'm surprised no one has mentioned that the Solr Cloud was taken out of beta! That is pretty awesome and we will be looking at this feature closely for sure.