Admittedly I've not read the entire thread fully so I hope not to repeat what's been said a dozen times already.
What irks me perhaps is prejudice. I won't claim to be entirely free of it myself, but this also is a fact that irks me so I think it is fair enough. There's always room for growth.
People who have encountered one character of a player remember the account name and proceed to refuse to play with any of their other characters if the first experience has been bad. I'm not even touching rp habits in this, but purely the persona of a character judging by what they were doing or saying at the time.
It also works the other way around. If one character of a player has been nice to someone, it doesn't mean all their other characters will be that someone's best friends. Then there's ooc tears or rage when things didn't go their way this time.
"That's that tavern rper's alt, booooooooring."
"That guy played a VILLAIN once, I'm outta here."
"Oooh that's that nice character's player, I'm going to glue myself to their EVERY character!! <3<3"
It is a very human thing to do and extremely difficult to entirely stay clear of. Still, a thing to consider perhaps and certainly one shouldn't take it personally if the encounter is entirely different from character to another.