Abilities is what does it the most for me.
Simple example, in 2K8 Derrick Thomas has the ball strip ability. I've played Madden for years and I've never seen Robert Mathis or Freeney come off the edge and perform one of those patented strip moves on the quarterback that we see on Sundays. Derrick Thomas will do it in 2K8. It's one animation, and it gets stale seeing the same one every time, but how the mechanic functions is what jumps out to me.
I know that if I can get Thomas a free lane to the quarterback's backside there's a good chance I will force a fumble in a realistic manner, not the AI's Dice Roll deciding my opponent's QB was going to fumble. It's a truly functional mechanic that deepens the game in multiple aspects. From a strategic aspect having Thomas' skill set means that my pass rush is that more dangerous, specifically because there's only 1 or 2 other guys that have the skill set, and neither of them can perform at the HOF level Thomas does. From a gameplay perspective, there's a learning curve on the ability; You have to be able to time it correctly and have the skill to maneuver around lineman and runningbacks to get the quarterback as efficiently and quickly as possible.
So when the animation does pull off because you've done everything right in not only a strategic sense but also a "stick skill" sense if you will, it's actually a really rewarding experience. It's one of those moments where you can look at your bro that you're playing against and say "I just did that, not John Madden".
People have their gripes about 2K but the only real problem I ever saw with the game was the learning curve is just too steep. We can sit here and complain about DB's staying home in goal line situations and psychic coverage but we're also talking about a game made in 2007, with relatively few resources, no NFL license, and the daunting task of somehow making a non NFL football game profitable without breaking the bank signing former players for their likenesses. The Mechanics of that game are difficult to master.
With an NFL license maybe 2K puts more into their product because they know now they can actually turn a legitimate profit. They don't have to spend their budget on just signing retired players and I imagine the budget would be bigger for a project that will easily be more profitable than the generic game.