Just like irc, /quit <reason>, i suppose it would be good to have the same command in-game.
also the thing should be like:
"Jaker250 disconnected. Reason: Leaving"

EDIT: Like if a person is leaving, he can use this command "/quit <reason>" and will be able to provide a reason why he is leaving... and if there is no reason, then the thing would be like "player disonnected. quit game". If there is a ping/crash, then "player disconnected. ping timeout/no response".
The "reason" thing should only come when you provide a reason, otherwise the same thing