Comparing defenders:
Fighter - Attack to mark, OA stops target movement, interupt attacks or shifting with MBA
Warden - Be adjacent to mark, interupt attacks with comparable to MBA vs. Fort that also gives C/A, can also react to attack at a distance (no damage)
Paladin - Minor to mark (a single target) at a distance with maintenance required to keep it up plus Divine Sanction, auto damage when mark is broken at any range
Swordmage - Minor to mark (a single target, more with feats) at a distance without maintenance, immediate action at a distance (10 squares) which requires the target hit, then either reduces damage, teleports the target and gives C/A or teleports the swordmage and allows an MBA.
Battlemind - Minor to mark (a single target, enhanceable with a power point) at a distance (maintenance?), opportunity action to follow shifter, reaction to a hit which deals auto damage.
The fighter is able to mark multiple targets if he can attack multiple targets, a warden can mark multiple targets by being adjacent to a number. The paladin needs to use divine sanctions to get extra marks up. The swordmage has the paragon and epic tier feats to get more marks at once (in addition to some powers). The Battlemind similarly is able to mark about as well as the close burst markers, getting only one in the "normal" mark burst. Unlike the swordmage or the paladin they have to be adjacent to get to do anything (that we know of, there may be another build option).
So, they mark like a swordmage, they have to be adjacent to the target like a fighter, they deal auto damage like a paladin (but it needs to hit like a swordmage), etc. The auto damage is great because (a) it only occurs on a hit and (b) it's after the ally is hit. So, obviously it should be better than the paladin's auto damage. The fighter (and warden) get to attack either way, so they still require a hit, but it's their hit that matters (which probably won't suffer from a mark penalty) and they determine the damage as well ... there is also the benefit of potentially killing the foe before it finishes the attack. Alternatively, the swordmage's most easily comparable power (aegis of assault) requires a second hit in order to deal damage. To balance this, a swordmage can be as far as 10 squares away when this happens, and be brought to the scene immediately.