What is the character?
Almost anyone can be a soldier, but this archetype is more than just a man (or woman) who picked up a sword in service of a nation. No, the soldier doesn't have a reason for fighting beyond simply fighting. He fights in the army, and if the army won't have him any longer he fights for another army, and if he can't find another army he becomes a mercenary. Some people can leave violence behind, but to the soldier, war is a comforting presence, something he understands and feels at home around.
As appealing as the aforementioned song might seem, it's not really fair to say that he's to blame for the conflicts that (inevitably) surround the world. It's rare that the soldier feels passionately about his side -- he feels passionately for the other people in his unit, perhaps, but he doesn't much care about the politics. He's told to go fight and possibly die, and he does so because the alternative is far worse than simple death. There's no need to resort to violence every time, but there's no need to be reluctant about doing so either.
What's the angle?
Pretty much every MMO is going to have countless wars either occuring, on the horizon, or just concluding when you create your first character. This gives him a clear reason for being an adventurer -- he's either fresh from the army or left the army for private work. And in the many games where adventuring-types fill out the ranks of armies, well, there's not that much of a conceptual leap to make in the first place.
Why he was in the army -- and why he left -- leaves plenty of space for filling in details. At the same time, it's important to remember that the archetype refers to someone who is a soldier no matter their occupation, not someone who just happened to serve at some point. He might have joined the army for any number of reasons, but these days, he's a soldier because that's burned into the core of his being. It's even possible that he's never been a part of a formal national army, instead working with a smaller private military.
Above all else, the soldier is pure. He fights because it's part of who he is. Any higher causes are more of an accidental result of circumstance brought on by who he is. To use a rather tortured analogy, a paladin fights to defend others, a soldier defends others because he fights.
What makes it interesting?
If you like playing competent characters, a soldier is usually a fine pick. He can be expected to know quite a bit about their field of choice, whether the soldier in question is smart or stupid. He also has a built-in reason for being on the road, taking part in combat, and being with almost any group of players. Even if your guild or group in question doesn't have a military wing, almost anyone can have a bodyguard.
Who or what your character is fighting -- or was fighting -- can make a huge difference in personality. He could be still fighting a long, lonely war that's either impossible to win or has been long since over, or he could have fought a short conflict that ended in a decisive victory for his side. He might have only seen small conflicts, and as a result be still remarkably unclear about what a prolonged war can actually do to people. For someone defined by war and armies, the background of a soldier still has many places where you can put in your own twists and flavors.
You don't need to feel constrained by a game's melee classes. Soldiers come in many forms -- medics, scouts, commandos, demolitionists, and so forth. A mage can be just as much of a soldier as someone in heavy metal armor, albeit a soldier with slightly less competence in physical arenas.
What should I keep in mind?
If there's a single word that sums up the archetype, it would be duty. A soldier will be ordered to walk into the line of fire and save as many people as he can, and he'll do it -- not because he wants to, but because he has his orders and that's what he needs to do.
Soldiers are accustomed to a structured environment. There's a chain of command, assignments, targets of opportunity, and strict hierarchies for everyone involved in a military unit. The purest soldier is neither eager nor reluctant to fight, doing so as the situation warrants. If your character belongs to a guild with any sort of in-game ranks, most soldiers will be very careful about observing it.
And while soldiers will not resort to violence to solve problems that don't require it, keep in mind that they can frequently ensure that problems solved with violence are very solved, very quickly. Some of them may not be terribly reluctant to bring it up. Just saying.
Let me know what you think of this particular idea, if you wish to see more of want to let the idea die a swift and unmourned death. You can do that in the comment field, or by sending a letter along to firstname.lastname@example.org. Next week, let's take a look at supplementing your roleplaying through means other than forum arguments.
Every Friday, Eliot Lefebvre fills a column up with excellent advice on investing money, writing award-winning novels, and being elected to public office. Then he removes all of that, and you're left with Storyboard, which focuses on roleplaying in MMOs. It won't help you get elected, but it will help you pretend you did.