There is
Project Reality for
Battlefield 2. It is an incredibly deep, teamwork-oriented and realistic mod that basically turns BF2 into a new game. Go to the site and download the game's manual to get an idea of its scope.
Its biggest problem is really not its learning curve, but the community - PR has its fair share of a lot of obnoxious, intolerant and grumpy players who may not be very friendly to newbies or even experienced players who make too many mistakes. Having to put up with them is a hurdle you'll just have to cope with if you want to play the game.
See also
Modern Warfare 3
.