Most sports can be broken down into three different elements; tactics, your physical condition and technique. So in extremely simplified terms; the mind, body and skill. Football falls directly in the middle of this triangle, making it a far more complete sport than Hockey.
On the physical side of things, you have to be much fitter, stronger and faster than Hockey players as you are constantly running and competing for two 45 minute segments. It also requires you to be far more agile and elegant as football is that kind of game.
About the mental part of the game, tactics are far more important. Games can be won and lost with the slightest tactical piece of brilliance. This in some ways is proven by the fact that football is the most competitive game in the world (source: http://www.newscientist.com/article.ns?id=dn8531). Then the variation between players is astonishing, no two players are similar, rather than the monotonous Hockey players. It is also much more of a team game, every player is deeply involved in the game and they all have to work together, far more so than Hockey could ever boast.
Then you move on the technical aspect of football. It revolves around the beautifully simple relationship between you and the ball. You can do so much with this perfectly rounded sphere of goodness, it is an art. Dribbling is an example, the way you can move the ball around, weaving between players with agility and skill. Then there is the many ways of shooting, depending on how you hit the ball with your foot. You can curl it with the inside or outside of your boot, you can hit it with astonishing power, or you can slide it past the keeper. Then there is the more entertaining chip, or through the keepers leg, or the athletic bicycle kick. This applies to so many other aspects of the game as well, heading, passing, defending, movement off the ball, positioning, kickups, flair, first touch etc. It is by far the most technical sport, or at least more so than Hockey.
It is also the most popular sport in the world with global appeal drawing in staggering crowds and supporters all the time. It has support Hockey fans can only dream of.
I would say the defining difference between the two sports is the variation and complexity they each offer. Hockey is a relatively simple game without much variation, while football is this amazingly complex and deep thing with so many subtleties, despite it's beautiful simplicity. I could talk infinitely about football.
I just can't express it's brilliance enough.