Second, there is a history's worth of weapon and combat styles to choose from, and every conceivable ethnicity and culture type awaiting the player. And they needn't be limited to historical accuracy, either, as dear old Ramirez attests -- your warrior can use any style she likes. After all, she's had centuries to learn, and the whole world to teach her.
And finally, third, and perhaps best of all, the opportunity to lose one's head -- and all of one's experience. What could be a fantastic element to a Highlander MMO, or its worst feature, would be for the victor of any battle between two immortals to win all the experience the losing character had built up over the course of gameplay. In the fiction, when an immortal loses her head to another, all of her memories are absorbed by the victor, and vaguely, presumably her style and abilities as well.
Now, imagine this as an actual gameplay mechanic: postulating a combat system that plays like Age of Conan's, in which skill is more important than stats, a player with the combined experience might not necessarily be the strongest character around. Let's assume that xp isn't used to buy greater health or armor, but better weapons proficiency and different styles that offer different advantages against opponents, and you have a truly level playing field where at any time the 'one' of 'there can be only one' can get their head taken off just as readily as anyone else. The loser would resurrect as a character starting over from the ground floor, level-wise, but in a system that rewards actual skill, rather than level, this is not so much a slap as it is a goad. Knowing your vanquisher's fighting style, you set out to get everything back that you've lost -- and stand a decent chance of doing so.
Pipe dreams, my friend, I'm well aware. But I'm always looking for the truly new ... and sometimes, the seeds of the new reside in the roots of the past.