Playing in character. There is nothing worse in my book than seeing the Chaotic Good Half Elf Ranger having some moral dilemma about killing orcs that also happen to be his/her preferred enemy. A character that has been a jokester the for adventure after adventure all of a sudden becoming straight laced for no reason. To me, that is important because then you know what to expect, like in real life with someone you know well and learn what to expect.
For me it is all based on consistency. Everyone should be able to play their character the way they want but when you don't show up to play that character it starts to affect my enjoyment of the game mostly because the Dungeon Master waits on you. Recently, I saw a Dungeon Master just disappeared, but its not the first time I saw that happen and it makes it twice as damaging compared to when a player is gone because the impact is on more players and the game. So I expect all parties to be consistent not just the players but the Dungeon Master too.