Trouble spots for Heisman faves

Marcus Mariota and Johnny Manziel are both Heisman front-runners, but both have weaknesses. USA TODAY Sports

Considering how far off the preseason radar the past four Heisman winners were, it may seem foolish to label anyone a "favorite" right now, but we know that some players definitely begin the season better positioned than others.

So here's a look at five of the early Heisman favorites. But instead of focusing on what makes each player a leading candidate, let's explore an issue that could keep that player from winning the trophy. And instead of just randomly picking five guys, I'm choosing the "M" quarterbacks, who happen to account for half of the 10 Heisman front-runners (based on current odds).

Braxton Miller, Ohio State Buckeyes

In Total QBR -- ESPN's new rating that considers every aspect of QB play within the context of game situations and adjusts for the quality of opposing defenses -- Miller ranked 29th in the FBS last season. And unlike the NCAA's pass efficiency rating, QBR includes rushing, which is a strength of Miller's game. Other dual-threat guys such as Johnny Manziel (first), Marcus Mariota (second) and Collin Klein (sixth) were rated highly by QBR. Why so little respect for the Buckeyes' quarterback?