Committing to an RPG
Joining an RPG may seem like a simple thing, but once you've made that first post, you are making a commitment.
A commitment? Yes. When you join an RPG, your characters will quickly become entwined in that RPG's plot. Thus, if you suddenly leave without mention, the RPG may slow or even come to a halt. That may not matter to you, but RPGs are an interactive thing, and the other people involved will not enjoy losing an RPG because one person didn't have the foresight to tell them they were leaving for Japan for a few weeks.
I understand that real life always comes before RPGs, and sometimes, notification simply isn't possible. This is okay; your fellow RPers will understand. However, merely leaving because you've lost interest, and not telling anyone, is not okay. If you're leaving for an extended amount of time, and you have the ability to say this, by all means, say it. PM someone active in the RP so they can notify others, send a quick IM... anything. But don't leave your fellow RPers in the dark, not when you can help it.