So besides what I'm seeing commonly cited as reasons to have one character instead of alts, I'd like to also contribute to the reputation argument. In most MMOs you can create alts, and its extremely easy to level up to cap. In FFXI the idea of leveling another character to cap was almost unthinkable because it took most people close to a year (or longer) to hit cap on their first job. (The fact that in FFXIV it's really easy to level is a problem in and of itself that should be addressed.)

As a result of it being so difficult to level and get through all of the quests (khazam keys, airship passes, limit breaks, AF etc) you needed to do before you ever got to 75, you had to rely on other people's help to get there. You couldn't be an ass to anyone because otherwise you'd just get no where. Because almost everyone just had one character, you couldn't be an ass on one character and then get help on another. Reputations were long lasting things in FFXI, and news of asshattery traveled fast on a server. It was a great way to ensure people didn't succomb to the lure of internet anonymity and do whatever they want because there were no consequences. In FFXI there were long lasting consequences, and thats why I prefer the FF MMO community. Its inherent in the single character paradigm. People get more attached to their single character and are also much less likely to jeopardize their community reputation with it. In short, it helps keep good people good, even when its a bad day.