I'm all for the "every class that was in a phb" (well, phb1) mantra, so off course I say class.
Of course you don't need a assassin class, to have in game people who professionally assassinate others. Every class can do that (though the "getting away with it too" part will be hard for some).
Now, this will sound incredibly odd, especially in D&D, with the semi official slogan "kill things and take their stuff".
I consider the primary difference between an assassin class and others, that it has a real focus on killing things.
Yep, that's a defining feature for me. Think about it, every class can kill, but none represents a person, who faced by opposition, how do I kill it quick.
Yes, the rogue can hit a vital point after setting up a favorable situation. But he does it because he's unable to fight in a more straight up manner and he can't do it reliably. But an assassin should be able to dish out terrible damage face to face and get even worse striking from ambush. The rogue kills one sentry from hiding and the fighter takes on three but does it loudly, the assassin calmly walks up to two sentries and with a smile killed the second before the first hit the ground.
Yes, fighters kill a lot of people, often a good answer to the tactical situation is "kill it, kill it quick". But that's the thing, the fighter sees the tactical situation in group battles, because that's his training. He fights duels, small group battles and full blown wars. The assassin eleminates individuals. He doesn't do so blindly, but very fokused.
Of course, there must be other qualities to an assassin.
I've also mentioned, an assassin needs to get away with it, too. This doesn't mean he's got an untouchable ac, because he's like a totally cool ninja. But he needs to have abilities that allow him to reach his target and get away, both when the target is and isn't aware of him.
But those are all abilities rogues have too, right? Yes and no. Yes, rogues have abilities that let them get in and out of places. But no, those aren't necessarily the same abilities assassins use. Assassins should get a choice from a huge choice of such abilities. Some are the classics, hiding, disguise, climb, open lock skills. But then let's add some (optional, of course) monk like wuxia movement mojo, so even revealed the assassin can cartweel across the banquet hall, put a fork through the chancellors juggular and the jump out that third story window to a soft landing. Lets also give the option for the assassin to pick up some mystic shadow power. You're killing people for a living, so why not make some deals with fiends too. And I bet assassins sometimes use bombs to, alchemy gives some great shennanigans for escapes and arrivals.
Another is that assassins know a thousand ways to kill you. The assassin doesn't just do a lot of damage, he knows many ways to do it. He can stick you with a knive in the back from hiding, or the belly from disguise or in the eye in a straight fight. He can poison you through your food, a hidden needle or a kiss. He can blow up your carriage, turn your bath into acid or electrecute you with your throne. If all the fails he smuggles flying piranas into your bathrobe. Mechanicaly, the assassin can just pick up a lot of different attacks, many of which I'd place in the realm of fantasy alchemy, as assassins would already be assumed to be an expert poiseners.
Opposite to the many ways an assassin can kill you, he is otherwhise not versatile. This sets my idea of an assassin really appart from the rogue, monk or ranger (the three classes I see him closest to). The assassin class has pretty much no ability outside of killing and getting away with it. Unless he really invests in backgrounds and themes, the has no idea how to defend someone, travel the wild, get on the good side of people, what monster does what and how to comprehend that feeling called love.
If we put that together we have a character with enourmous offensive power, but little resilience, and great mobility. He can have all kinds of mystic mojo through his class but doesn't have to. He's deadlier than a rogue or ranger, but lacks their mastery of skills.
