PB_BI (9/26/2016)You can directly query Tabular cubes with both MDX and DAX (SQL 2012 SP1 CU4 and greater) but Multidimensional cubes will only accept MDX.
Small correction to the above. You can query a multidimensional cube with DAX as well (called DAXMD). This functionality has been available since SQL Server 2012.
Reference: http://sqljason.com/2012/12/querying-attributes-and-measures-in-dax.html