Huh. I see a much tougher Denver team than a lot of both Patsfans and Broncos fans alike.
I think that, especially early in the season, perception is often based too much on the W-L record than the overall quality of play. I know that Bill Parcells was fond of saying you are what your record says you are, but while that's a useful mind-set for a coach to instill in his players, it's not really sensible when it comes to objectively evaluating a team.
If you want to know how good a team really is, you have to look at their performance at the drive, down series, and play levels. The Pats would not be significantly better team if Gostowski had made that FG against Arizona and Justin Tucker had missed his, yet the perception of them as a 4-0 team would be radically different.
While Denver's two losses may not have been quite as close, they did come against two of the best teams in the league, and it's not really accurate to say that their 2nd half comebacks were just garbage time scoring. In both cases, the Broncos pulled within one score with enough time left on the clock to get Manning the ball back if the defense could make one more stop. In both cases they managed forced their opponents to make tough conversions on 3rd and 5 to avoid going three and out.
Even more so than the Pats this season, Denver has dug itself holes by getting off to slow starts. Fortunately for Denver, this tempts the opposing teams into playing towards the Denver's strength, which has been run defense. Their pass rush has been solid, as has been their downfield coverage, but it looks like they struggle preventing YAC on short passes. Then again, having played against teams with downfield receiving threats like Mike Wallace, Andre Johnson and Roddy White, my guess is that has a lot to do with game-plan, and that they'll take a very different approach to the Pats.
Meanwhile, a lot has been made of Manning's potential loss of arm-strength, but I don't really see it. When he's got a man open deep, his passes have the same taught arc as always, and he's made enough hard throws into tight spaces to show that he's physically capable of it. When I've watched him, it's looked to me like he's more indecisive than before and isn't as confident that his receivers and he are on the same page, and this is leading to a hesitancy in some of his throws that can look like lack of arm strength. When the Broncos have gone to their no-huddle shotgun offense, and Manning gets to make more of the sight-reads for his receives, they seem to click into gear.