FAVORITES. STEELERS LAST YEAR: 11-5. Playoffs: Super Bowl champs INCOMING: Why change a good thing? For the second straight year, the Steelers signed just one free agent, safety Ryan Clark. Top pick Santonio Holmes, the WR from Ohio State, is still learning the offense. OUTGOING: Short-yardage and goal-line situations won’t be the same without RB Jerome Bettis cranking up the engine. Versatile Antwaan Randle El is now with Washington. The Steelers should weather the loss of S Chris Hope to the Titans and DE Kimo von Oelhoffen to the Jets. MAIN MAN: Everywhere-safety Troy Polamalu is the heart of a defense that is the heart of the Steelers. Not since Lawrence Taylor has a player given offensive coordinators this many headaches. STRENGTHS: QB Ben Roethlisberger’s recovery from his nearly fatal motorcycle accident has quieted fears about the offense, where RB Willie Parker can only get better. Few defenses are as adaptable or present more problems as this zone-blitzing unit. WEAKNESSES: The O-line lacks depth and WR Hines Ward has been battling a preseason hamstring injury, spotlighting a lack of depth at wideout. BOTTOM LINE: Don’t look for Super Bowl hangovers with this mentally tough group. Now that the thumb’s covered,…