Exchange 2013 introduced the CU updates which are actually full versions of Exchange will all the updates rolled into the installation. Various updates will will updates the AD Schema and affect .net versions and/or installations.
If one takes into consideration the dependencies of the various CU versions, you can jump to the latest CU without the requirement of performing incremental upgrades.
Paul Cunningham has been a great resource for Exchange admins and has written an excellent article on the implementation and considerations.
Here are the top warnings,
… “Caution: a cumulative update is a full reinstall of Exchange Server 2013. If it is interrupted, or fails part way through the installation, you may need to perform a server recovery. There is also no way to uninstall a cumulative update.
Note: Exchange 2013 cumulative updates stop the “Microsoft Exchange FrontEnd Transport” and “Microsoft Exchange Transport” services during the pre-requisites check. If you do not proceed with the installation you will need to manually restart the Microsoft Exchange Transport service. … ”