Fun is number one, but for me fun is made up of the following attributes: Style (I won't play clunky classes ever and want my skills to be flashy without being obnoxiously gaudy), story (The character must have some compelling motivation), potential (If I get a decent range for them, they shouldn't be lagging far behind every other class and should also be useful in all party scenarios) and skills (I don't want too many skills, just give me a few that go great when they're combined and that's alright).
...basically, Phantom.
...basically, Phantom.