Athleticism is defined as the ability to apply strength, speed/agility and endurance and.or more simply: being good at sports. http://dictionary.reference.com/browse/Athleticism
. All the work ethic in the world would not matter if you are not athletic enough to apply those physical attributes to the goal (in this case football but really any sport).
Now, Wis may define athleticism differently, especially since the attributes which make up athleticism are each used seperately, but I think most coaches in real life when drafting (combine measures athleticism which is really potential)., or recruiting (when they go to see games they look for skills but they also look at potential which would be the attributes above like speed, agility, strength, in football but also eye hand coordination, jumping ability, flexibility or any other physical trait related to sport) look for talent/potential as much as skills. Wis captures this feature well with the scouting, but the reasons for the high potential vs low potential is a bit confusing.
The most obvious example of this is in the NFL when guys are drafted that did not play football based on their athleticism alone. COaches believe with the right work ethic and coaching, the athleticism is the Xfactor that defines potential. The reverse is true too - guys with all sorts of college awards and stats may not get drafted because they are not athletic enough for the pros, despite their college success.
So to answer your question, a cursory glance at the available attributes would show work ethic as the desire to improve, while athleticism seems like it would be the ability to improve. All of this is based on real life sports though, not simulations. That is why I was asking what it is based on here in this game.
Sorry if the answer seemed long winded but I had plenty of time to ponder this while wrestling in college. A full day tournament might consist of 30 minutes of wrestling total, while the rest consisted of watching others and determining what made them have success or failure. Sometimes the best recruits with the most desire never improved despite hours in the gym and the mat, while others got better just by thinking about wrestling. Conversely, sometimes guys that had all the ability and talent never lived up to their potential because they lacked the desire to improve. So I guess I am saying work ethic(desire) x athleticism (talent) = potential.
Hopefully that makes some sort of sense!