Unneeded sacks - Times when the protection was considered good enough and there was a significantly open player or space to run to avoid a sack but the QB's indecision or the decision to run in the "wrong direction" caused a sack that isn't the fault of the blockers nor did somebody come unblocked. Call this the Bledsoe stat.
Bad decisions - When deciding who to throw to (or not to throw to) or running the ball results in a negative play (negative yardage or INT/Fumble), or the extreme probability for one exists (the ball is tipped in the air for example).
Avoiding sacks/tackles - The movement behind the LOS that allows a QB more time to throw the ball when a sack appears imminent, or when running, causing unblocked tacklers to miss or whiff.
Unforced errors - Fumbling the ball untouched or due to improperly securing the ball (holding it away from their bodies), prematurely throwing away the football (no defender unblocked within 10 yards and no blocked defender within reach), or poor passes that led to incompletions/INTs or the extreme probability for one exists (the defender drops the INT).
Deep passes - Passes that are caught 30+ yards from the LOS.
Medium passes - Passes that are caught 10-30 yards from the LOS.
Short passes - Passes in the "flats".
Receiver aided positive plays - The amount of yards / passes caught / INTs avoided due to an excellent play by the receiver that hid the bad play of the QB. For example, a 5 yard slant TO takes 40 yards to the house after making 3 defenders miss isn't the same as Campbell hitting a running Santana from 40 yards away in the endzone. This also accounts for excellent one-handed catches that receivers are not normally expected to make or when the receiver has to knock the ball out of the defenders hands to avoid an INT.
Receiver aided negative plays - Errors attributed to the QB statistically that were not due to his bad play. For example, if the ball hits the receiver's hand and pops up and is then INT, it's probably not the QB's fault. Also, all dropped balls goes here.
Forced Errors - When under heavy pressure, the QB makes a bad decision as defined above.
Throwaways - Self-explanatory
This is probably the hardest position to analyze, so bear with me while I refine my criteria.