His turnovers and inconsistency comes to mind.
In wins this year (6 games) he is 127-181 (70.2%) with 244 Y/G, 8 TD's, 2 INTs and 2 fumbles lost for a QB rating of 102.5
In losses this year (6 games- actually 5.5) he is 87-143 (60.8%), 152.3 Y/G, 3 TD's, 8 INTs and 2 fumbles lost for a QB rating of 63.1.