The biggest difference between 2008 & 2014 was the new cardinality estimator. First off, make sure the database compatibility mode is set to 120. If it is, you're using the new estimator. That can cause problems for some queries. You can turn off the estimator by changing the compatibility mode, but you're better off identifying if there are queries that need to be turned off and then applying traceflags. You can read more about it here.