McMaster has been quoted as believing that Islam is not involved in terrorism, that there is nothing wrong with un-masking Americans, and that Susan Rice remains worthy of keeping her security clearance.  Those beliefs may have been appropriate for the Obama Administration, but they are not appropriate for the Trump Administration.
Probably the strongest complaints concern McMaster's purge of those in NSC who don't share those beliefs.  
The strongest defense of McMaster, that has been made thus far, is that he was unaware of the purge, but, "unawareness" is hardly a qualification for the cabinet post that McMaster holds.  Neither are beliefs that coincide with Obama policy.