The 2.0 milestone will mark the beginning of the second generation of Doctrine. The primary goals are:

* A more robust, simplified programming model with a cleaner API
* Clear separation of layers (DBAL/ORM)
* A new, simplified and unified Behavior implementation.
* A smaller core library (i.e. Behaviors as separate downloads)
* Improved inheritance mapping
* A new DQL parser

Secondary goals are:
* Improved Performance

You can read a more completed list here: http://trac.phpdoctrine.org/wiki/Doctrine2.0