What are the low hanging fruits of a SQL Server 2014 migration?
In 2014 you get the new cardinality estimator which could see your queries get better plans and therefore execute more efficiently. 

A good reason to upgrade from 2008R2 is that it is on extended support meaning it is unlikely to receive anymore updates and only SQL Server 2012 SP2 + versions are able to operate with TLS 1.2 see here