32-bit SQL Server. SQL Server 2016 is 64-bit only. If for whatever reason you’re running on a 32-bit architecture, sadly you’re now out of luck – 2014 is the end of the road. On the bright side, there’s probably some new hardware in your future!
Compatibility Level 90. If you’re using compatibility level for backwards compatibility, the oldest available version in SQL Server 2016 is 100, which corresponds to SQL Server 2008. Compatibility level 90, SQL Server 2005, is no longer an option.
Bob also covers a few deprecated features, none of which (hopefully) are in regular use in your environment.