It would be nice to have an option to automatically quit the client after the agent has been idle for a configurable amount of time.
Some poeple fall asleep or forget that they still have their client running which can be annoying for someone sending them IMs and getting no answer, which leaves to wonder if that someone isn't talking to them anymore or what happened. Computers also consume more electricty when the client is running, and that could be avoided. It would also unburdon the SL servers and other resources from obsolete connections when people have the option to get automatically logged out when they're not using their client anyway for a while.