Frenchie-
I think that there are a few reasons and perhaps in some cases they apply more to one team than the others. For my money it boils down like this:
1) Winning the trenches. There really is NOT a position in football that you can ignore but offensive line is a great place to build; you can have all of the great skill players in the world but if no one blocks, you're hosed. Tom Brady looks awful pedestrian under pressure. Detroit has built a solid offensive line and a dominant defensive line. Control the line of scrimmage and you control the game.
2) Franchise QB. I don't know that SF or Buffalo have one but Detroit does and none of the QBs playing for these teams are making huge mistakes.
3) Multi-dimensional offense- You'll notice that all of these teams can find balance on offense. Sure, the Lions like to pass more than run (as most teams nowadays do) but they have to be able to run the ball enough...especially when they have to get a yard or run clock. They don't rely on one weapon.
4) Athletic defense- San Francisco is winning mainly on defense and it's one that's been built over time.
5) Learning how to win. I know our previous coach is not the most popular guy but teams have to learn how to win and finish off opponents. How many times have we seen the Browns play well enough to win but come up short? It's about limiting mistakes and executing when it matters most.
Of course, the experts always seem to say that if you're building from scratch the importance of the players acquired starts with QB, LT, pass rusher, MLB...that's not a bad way to think about it as those are the highest impact when building a core. Ask yourself: Does SF/Buf/Det/Oakland have those? Let's look
SF: Alex Smith, ?Aldon Smith, Staley, Patrick Willis (3 of 4 at least)
Buf: Fitzpatrick, ?Shawn Merriman, Demetrius Bell, Nick Barnett (2 or 3 of 4)
Det: Stafford, Vanden Bosch, Jeff Backus, ?Steven Tulloch (I will argue 4 of 4)
Oak: ?Campbell, ?Veldheer (has been good), Lamarr Houston, McClain (at least 2 of 4)
Where do the Browns sort out?
?McCoy, Thomas, ?Sheard/Mitchell, Jackson. That's 2 of 4 by this man's estimation.