Performance testing has suggested that hash-based index intersection, implemented by the AND_HASH query execution stage, does not offer a significant performance advantage for test workloads. This is due in part to SERVER-12923, which describes how query optimization can suffer for plans that are not fully pipelined.
Based on the results of this testing, we should disable hash-based intersection by default. It will be possible to re-enable AND_HASH at run time via:
Sort-based intersection will be on by default and will not be affected by this change.