I think I've identified the biggest flaw with Assassin's Creed's multiplayer, and it's the exact same as every similar game out there.
The multiplayer is built on a distinct style of gameplay. You're hunting someone, someone's hunting you, blend into the crowd, and strike silently for the highest score. Like in most such games, people are too stupid to play it that way and you get a lot of degenerate tactics that drag the whole game down. The two biggest ones are running and batman. Some people run everywhere and just leap on their targets. Not only does that give you the worst score and make you really conspicuous to your target and pursuers alike, it also really fucks up everyone else's playing too. Getting assigned a runner as your target wastes your time because you have to chase them around the map, and lowers your score because you have to stoop to their level to beat them. Luckily I'll have the hidden gun perk soon which lets you just shoot the fuckers.
Batman is the other bad one. Stand on the roof all day and jump down on your guy when you see him. It's impossible to sneak up on a batman, because you have to go up to the roof. It's not quite as bad though, because they're also very conspicuous.
The runners are the real problem. Until you get the hidden gun and can shoot them they're really fucking irritating. Once I went from first to third place because it kept assigning me a guy who ran everywhere.