A change introduced in Exchange 2013 that many administrators are not aware is the fact that with the Enterprise Edition of Exchange, you can now only mount up to 50 mailbox databases per server, a reduction in 50% from the 100 with Exchange 2010! The limit of the Standard Edition remains at 5 databases.
Highly available and resilient environments might have some problems when migrating from Exchange 2010 if they have servers with more than 50 databases (in big environments with 3 or 4 copies of each database it is not that uncommon). Therefore, a complete review of the current database layout might have to happen.
But why this change?! Basically it was introduced in order to ensure a good performance from the mailbox servers. Some of the reasons behind this change are the improvements made in some areas, which mean the mailbox servers consume more memory now... For example, Exchange 2013 uses Search Foundation instead of MSSearch in order to be consistent with SharePoint and to allow discovery searches across e-mail and documents. Search Foundation uses more memory and it seems it can take between 10 to 15% of available memory on a mailbox server.
Another change is the move of protocol handling from the Client Access Server [CAS] to the Mailbox server. It helps make the CAS more stateless and not so dependent on a particular mailbox server but it also increases the memory use on the mailbox server...
Note: the limit of 16 mailbox servers per DAG remains in Exchange 2013.