Teams with good offenses can use a more aggressive style defense and take chances knowing or feeling their offense can get the points back if they give up a big play here and there. We'll see if the Broncos plan to do that or not, at least at times.
The Bronco defense over the last few years it seems has not been good at causing turnovers, pass rush has been spotty, giving up huge runs, and the inability to cover TE's or RB's in the passing game. Getting off the field on 3rd downs is another area of improvement, though as bad as some defenses the Broncos have had in recent years, teams didn't seem to get into many 3rd down situations since could get such large chunks of yards on 1st and 2nd down.
If the Broncos want to be a serious Super Bowl contender, then a top 10 defense is needed and really, has to happen. Manning is 36, coming off not playing a season and having multiple surgeries on his neck. Elway at 36 needed a complete team to win a Super Bowl. I would expect the same for Manning.