I honestly think the big difference is the 'your character is your account' mentality. When you can go between characters it makes everything feel effervescent or less important on alts. With one character -for most players- there is your story with your name and face attached and that sticks. Throw in touches like characters regarding you differently if you did certain questlines or have a certain grand company rank means you are less a generic jobber but someone that earned a rep to the characters. If you have to share the load across a dozen characters that gets kind of lost. Even when WoW was my most hardcore gaming timesink in BC i still looked back on the same idea in FFXI and thought it was better.
Its a shame they started to think about this in WoW during wrath, originally a character you had at 70 would die fighting the scourge and on the login screen they would now be a new character in the dk starting experience. Thats long gone though.