Roethlisberger has never been very quick on his feet; he's just been hard to bring down. Aaron Rodgers and Tony Romo are more mobile than Roethlisberger is, but once you get to them, they're easy to take down. Roethlisberger looks kind of clumsy when he runs, which is why his 30-yard TD run against the Browns in 2007 is one of the most memorable plays of his career. (I was at the game but missed the run because I'd just left the concession stand with a fresh bucket of chicken wings when it happened. I heard the gradual crescendo of the crowd, though.)
He has some extra wear on his body, but that's why the Steelers invested a first-round draft pick in 2010, a second-round draft pick in 2011, and a first- and second-round draft pick in 2012 on offensive linemen. And everything I've heard about Todd Haley's playbook is good too; he wants to involve Heath Miller in the passing game more, and the Steelers just drafted Chris Rainey, who's sort of a Darren Sproles-type player. These things should greatly reduce the wear and tear on Roethlisberger's body in the next few years.
"We're a game-plan offense, and we're going to try to tailor our strengths against their weaknesses."
-Todd Haley, current Steelers offensive coordinator
"We're not going to change what we do regardless of who our opponent is or who's injured."
-Bruce Arians, former Steelers offensive coordinator