The fighter has a basic ability to mark targets; after that, if they attack someone else instead of him, he gets either a free attack or a bonus to hit. I actually consider this an extension of a big part of the fighter's role in classic D&D, and it helps make the fighter more useful in combat, asde from his ability to do massive amounts of damage.

Also, as someone who plays a lot of mages, I am naturally in favor of this idea. ;')