In addition to the points mentioned by Tim, I want to give more info related to new mailbox agent.
With the new Exchaneg Mb agent we can achieve below
1) In new agent data will be associated vitual client, this will give a flexibility of adding multiple proxies based on growth of organization, whilch will be a horizontally scalable environment.
2) To improvize the performance we implemented multi streaming by providing number of service accounts.
3) New agent desing is user centric, user level archive /retention policies can be applied.
4) New indexing framework infrastructure requirements are very less with respect CI engines, with new index server, number of objects can be populated in single index is very high 10times than older CIserver.
5) Content Indexing can be limited to group /single users, where as with old agent for single user we need to CI all the jobs.
overall new agent will be highly scalable and meets user centric requirements.