I usually get sad lol at least when players just disappear without a trace. If they give a reason or at least admit that they don't have interest in it anymore, it's all good actually. Then there's just the question of how to deal with their character disappearing and not the mission of, heh, hunting players down and trying to get them to post.
Sometimes the plot can be retconned so that the other characters pretend that the leaving one never existed, but that's not really elegant imo.
Often seen is the leaving character becoming a NPC for the GM to handle for a while, or even for the other players to use, while they either keep their importance for the plot through NPCing or are faded out from the story.
Another way is to have the character killed, I guess that's quite common, right? Depends on the RP of course. But I don't like to kill off people so I most often go with the NPC fade out style :p
Once when I was the GM (in checkmate actually) I took over a player's character completely when they left, but it was difficult to play with a person you hadn't created yourself.