I think that Denver is a legit playoff contender. They need a good QB, a starting RB, and a competent OL to be in serious consideration for the SB. Sieman needs to step up, not to Rodgers level, but needs to be more than he's been.
Defense doesn't win championships - except for 2000 Ravens. Offense wins in the modern era, and it helps a LOT if you've got a good defense. I see a Denver offense that is about as competent as Dilfer's in 2000 but the defense isn't of the same caliber - which, was one of the greatest of all time.