"Oops GTG TTYL bye" – heard that one before, in the middle of an instance? You don't want to be that guy.
Today's World of Warcraft is a far cry from Tomb Raider or Lego Indiana Jones, which can leave you writhing in your chair for what seems like an eternity before your next save point. For an MMO, WoW's gameplay has become reasonably bite-sized – so don't get caught virtually chewing with your mouth open by dropping out of groups early.
True, many instances can be completed so quickly and many players are so experienced that dashing through a quick instance can seem almost trivial. But a group is a social creature, and accepting a group invitation means accepting a reasonable responsibility to treat the other players' time with respect.