Over the off-season I had grandiose plans to improve the QB rating system as the current system is lacking on many fronts. However, things didn't pan out the way I wanted... That said, apparently ESPN read my mind and did all the work for me:
QBR looks at every facet of quarterback play, from passing and rushing to fumbling and taking sacks, and allocates credit or blame to QBs according to how each and every play they make contributes to their team's success.
And if you care at all about stats, that's the key: QBR finally brings all the advantages of win probability to football. To determine who the most valuable player in the NFL is, compare the average value of, say, wide receivers and defensive backs, or figure out whether it's worth it to onside kick, you need a system that tells you how much various plays affect a team's probability of winning a game. QBR does just that for quarterbacks. A 5-yard completion on third-and-3 is much more valuable to QBR than a 5-yard completion on third-and-15 because, in real life, it gives the quarterback's team a much better chance of coming out ahead.
QBR is scaled from 0 to 100, with 50 representing league-average performance. For a single game, a rating in the 90s is terrific; last year, Vick's six-TD Monday night symphony against the Redskins topped the charts at 99.8. For a season, any QBR above 65 is Pro Bowl-caliber, and Tom Brady led all starters with a 76.0 QBR in 2010.
When I have some time coming up, I'll run some CFL numbers and see what the ratings look like.