With revenue sharing, a salary cap, and a draft where teams select in inverse order of their finish the previous seasons, no team in theory will have a significant advantage over another in the NFL.
But the NFL, like all sports, is a zero-sum game. One team has to win, another team has to lose. Unlike baseball where the teams with the deepest wallets can purchase the best talent available on the market, to succeed in the NFL you must get a majority of your draft picks right, sign the right free agents, and be sure to lock up your homegrown superstars in order to contend.
Hiring the right head coach and drafting the right quarterback are also important. As is stability, since odds are high-caliber free agents will be more likely to go to stable teams.
This slideshow is dedicated to the 10 teams in the NFL that over the last few years have failed to do these things.
Stupidity is the one thing that separates the NFL haves from the NFL have-nots. These teams have been the NFL have-nots and will continue to be unless they undergo change.
You'll notice some of the teams on the list actually have had recent playoff appearances or are doing well so far this year. You will also notice that some teams that are doing poorly aren't on the list (i.e., Indianapolis). But bad luck can change from one year to the next, which is why I left some poorly-performing teams off the list.
But stupidity will hinder your franchise until you remove that element.