Growing up in Washington (not D.C.) and living in Colorado, I have watched both of these teams nearly every week. Here is my takeaways. Please feel free to disagree, but these are observations. I was a recruited high school offensive guard and inside linebacker 20 years ago (knee injury missed my senior year, so no college for me), so I definitely come into things with a bias/understanding of the impact of the Offensive Lines.
Just to expound on my matchups and I how I see things playing out.
In watching the Seattle-Denver pre-season game, the one thing I noticed was that Peyton (he isn't Manning, is he? He's Peyton) got very frustrated, something that does not usually happen. If Seattle's D can frustrate him, it will be a wild game.
Seattle's D-Line = Denver's O-Line - Because they have so many amazing edge rushers (Avril, Clemmons, Bennet, Irvine, and Bryant) and a ton of interior wide-bodies (McDonald, McDaniel, Mebane, and Hill), they will be able to wear down the Denver O-Line. Look for big things in the 2nd and 4th quarters. Also, Manning is very not mobile. I do not think he will throw more than one INT, but two is possible. Look for a few sacks. Watch for Denver's O-Line (and pass game) to be effective in the 1st and 3rd quarters, when they are rested. This, to me, may be the key to the game. If Denver's O-Line can give Peyton time to pass, Bronco's win. If he is constantly hurried and frustrated, Seahawks have a greater chance.
Seattle's Secondary > Denver's Receivers - This is mostly just the pure receivers, but the 5 DB's/Safeties that Seattle runs out there are the best I've seen since... shoot, Tampa in 2002? And that is very close. Welker is a wildcard. I cannot wait to see how they try and defend him. I expect him to catch 10 or so passes, because the intermediate threat from the DB's are big. Denver can exploit most of Seattle's LB's in coverage.
Denver's D-Line >> Seattle's O-Line - Seattle's O-Line is a mess. Yeah, they went half the season with second and third stringers because of injuries to both tackles and their center, but even at their best, they aren't great. Very average. Okung is really not the LT they had hoped for. He did not handle Smith well at all. Might be lingering injury issues, but Denver's D-Line will mean there will be 3-4 sacks on Wilson. If Wilson can not roll out or run for ~40-50 yards, Seattle will struggle offensively.
Denver's Secondary > Seattle Receivers - Denver's secondary wins this only because Seattle's receiver corps is weak. Losing Sidney Rice really had an effect the last few games (see Wilson's reduced passing numbers, they are not from a vacuum). Sure, a rusty Percy Harvin will help, but it is not a panacea. It will really come down to how much rust Harvin has. As well as how much they use Champ Bailey on Tate and Harvin. Champ is.. well, old. He has lost a good step and a half. He should camp on Doug Baldwin and Jermaine Kearse and call it good.
Seattle's Running Game > Denver's Run Stopping - Yes, I have major doubts around Seattle's O-Line, but Marshawn Lynch's Yards after Contact was 77 yards out of 140 yards. He flat out gets angry when tackles are attempted. Look for Lynch to be a very angry runner.
Seattle's Run Stopping = Denver's Running Game - Denver is definitely a pass-first team. If weather is poor, look for good matchups here. I call it a push, but it might be in Seattle's favor. Seattle's LB corps are typically pretty good at stopping the run, but Kam Chancellor at Strong Safety is a terror. He is the size of a LB at 6'3" 232lbs, but with excellent speed for a safety. He is a run stopper. Excellent on runs to the outside. If Denver keeps it up the middle and keeps up the play action, it will be a push.
Seattle Special Teams > Denver's Special Teams - Seattle has had 82 yards of punt returns of the season. This is a return of 3.9 yards / punt. Seriously? Do not look for any sort of run back for a TD against Seattle Special Teams.
Denver QB >> Seattle Defense - I don't know if there has been a smarter QB than Peyton Manning. He really is the smartest guy on the field. If he gets creative, he can single-handedly turn the tide for the Denver receiver vs Seattle secondary battle. If he does, and Seattle doesn't adapt well, look for a Denver win.
Seattle QB = Denver Defense - Russell Wilson has raw talent, leadership, and charisma that drips from him. Give him 10 years, and we'll see what happens. But for now, because of a simple lack of experience (and the highly suspect Seattle O-Line), look for Seattle to be a highly balanced run:pass offense. If they can find decent success on both, look for 20+ points. If they stall on one, and the other cannot make the Denver Defense pay, look for low scores.
John Fox/Peyton Manning = Pete Carroll - I call this a push, mostly because John Fox is not as good on offense as Peyton is, and Pete Carroll, while good, his Offense Coordinator Darrell Bevell lacks imagination. However, whoever has the best plan (weather permitting) will probably win.
The Weather Factor - if its bad weather, look for Wes Welker and Marshawn Lynch to have huge games. Julius Thomas could be the difference maker, but KJ Wright may just shut him down (he did against the Saints' Jimmy Graham). I cannot wait to see this matchup.
My keys to the game - Denver's passing game, Seattle's balanced offense. Both need to find some success. If it becomes a defense-dominated game, it will be Seattle getting its first Super Bowl. If Seattle's Defense scores, look for a Seattle win.