1. Scoring - There are multiple ways to score (touchdown, field goal, safety, extra point, two point conversion) and each of them has their purpose within the game. And the frequency of scoring is a happy medium between sports like basketball, where it happens every minute, and soccer (where 0-0 and 1-0 scores after 90 minutes are all too common).
2. Statistics - Football has a ton of statistics. And each position has its own set of statistics that you can pore over and analyze and argue with your friends until the matter gets settled with slap bets or an exchange of legal tender.
Yards per game (offense), yards per game (defense), passer rating, passes defended, interceptions, sacks, tackles for loss, receptions, yards after reception, yards per carry, field goal percentage, yards per attempt, points per possession (offense), points per possession (defense), all of these things are statistics that get kept, analyzed, and discussed, each bringing a unique perspective to the game and how teams stack up against each other. It's endlessly arguable, which is great for the 24/7 sports news cycle.
3. Parity - Unlike some leagues like Major League Baseball or the Premier League, there is a salary cap in football and a revenue sharing agreement between all the franchises in the league. This makes every team competitive from season to season, and football dynasties last about 3 seasons before they fall apart. Compare that to a team like Manchester United, who's won the Premier League 12 out of 20 years, the NFL's parity keeps things fresh and exciting as dominant teams come and go, giving every dog (and their flea bitten fans) its day.
4. Time Commitment - Teams play every week (except for their 2 bye weeks) for 3 and a half hours. Unlike the MLB (where teams play every day except for 1 day every 2 weeks for 6 months) or the Premier League (whose regular season is literally 3 seasons long), the NFL operates on a 4 month regular season with a small 2 month postseason. Even casual fans can insert themselves in the conversation with the diehards just by watching 1 game a week. It's an all inclusive sport where you don't need to commit an inordinate amount of time to enjoy the game, but you can if you want to.
5. Strategy - Football is the one sport where coaches can draw up a plan for each play of the game. Things like formations, matchups (and its importance in creating mismatches), route running, blocking schemes, all of this is considered when the two sides lineup. The truly great plays in football combine both a mix of raw athleticism and a perfectly executed plan drawn up by the coach. And when things unfold exactly how they should, with the timing right, the execution perfect, big things can happen. And any play from anywhere in the field can result in a score.
6. Sheer athleticism - Sure, in a given game, there's actually only 11 minutes of actual football being played in an event that takes about 3 and a half hours, but the athletes who play them are going full throttle the entire time.
Players literally fly towards each, and big, huge athletes are much faster at sprinting than you might think. A wide receiver climbing the ladder to pull a ball from 12 feet in the air while being covered by 3 defensive backs into his arms is one of the most physiologically impressive things a human body can achieve.
7. Embrace of Change - The NFL is the most forward thinking sports league in terms of embracing and using technology to enhance the product. This goes from production value (the average NFL broadcast requires about 7 heavy duty trucks crammed full of video and sound equipment) to affecting the game itself (via instant replay challenge). And the NFL is always quick to change the game up in order to make it a more dynamic, compelling product. Compared to old, stodgy sports like baseball and soccer, NFL football is always new and it's always on the bleeding edge.
8. Badassness - I don't think there's a single still image in sports that looks more intimidating and menacing than a sideline shot of the line of scrimmage.