Plane Sailing said:
It is a major additional complication (much more complicated than simple status effects since you've got to remember who caused the mark). I'm not yet convinced that it offers anything really worthwhile to the game.
As ainatan asked up near the top of the thread "what is the ultimate purpose of marks?"
I disagree that keeping track of who marked you is "much" more complicated than status effects. It happens so often you get used to checking every round "Who am I marked to? Ok, I attack him to avoid the -2."
As for its purpose. It is their to facilitate the defender role and allow an archetype that doesn't work that well without it: The heavily defensive fighter. You know, the dwarf in full plate with a shield who is as tough as nails and can take a bunch of blows before dying. IF you can hit him.
This is an archetype that is strong in MMOs in particular, since almost every last one has a class whose job it is to get monsters to attack them and to be tough enough to take it while everyone else hurts it. However, I've seen people try to make that character type a number of times in 3e/3.5e.
The problem with this archetype is that very quickly you realize that it doesn't fit in a group that well. It can stand there and up its AC a lot and have a lot of hitpoints but unless it is in very rare circumstances, there is no reason at all for the enemies to ever attack it. So, when enemies are in melee with the rogue sneak attacking them and the dwarven fighter who keeps missing due to his strength being low and the penalty to hit due to his tower shield and the fact that he spent his money on a better magic armor than weapon...well, it's rather a no-brainer who you are going to attack.
The "best" solution to this from a mechanical point of view is probably some fighter ability that just MAKES the monsters attack you like they have in most MMOs. However, most people dislike the idea that the fighter can somehow "charm" the monster into attacking them. So, you need to find another answer or completely reject the idea that a martial class can ever "protect" the rest of their group.
The answer they came up with was simply to give monsters(and vice versa when used on players) a mechanical reason to attack the creature that marked them without "forcing" them to. Do you attack the rogue and take -2 to the attack roll but stop the sneak attacks from coming or do you attack the fighter and not take any minuses?
So, what it adds is an interesting tactical choice that lets there be a reason for a group to have a fighter instead of more rogues or rangers.