I read a completely different explanation of QBR from your guys at Football Outsiders.
Total QBR incorporates information from game charting, such as passes dropped or thrown away on purpose.
Total QBR splits responsibility on plays between the quarterback, his receivers, and his blockers. Drops, for example, are more on the receiver, as are yards after the catch, and some sacks are more on the offensive line than others.
Total QBR has a clutch factor which adds (or subtracts) value for quarterbacks who perform best (or worst) in high-leverage situations.