This is a better list of changes from prior stable releases, rather than just the prior beta:https://www.elastic.co/guide/en/elasticsearch/reference/2.0/...

Read through the series of pages to understand all major changes.

Those are the changes in beta one, not sure if it's complete but it will give you an idea:

https://www.elastic.co/blog/elasticsearch-2.0.0.beta1-coming...