So I came to WoW from a text MUD. You may have heard of it. It's called Achaea. In Achaea everything is "in character," or IC. Nothing is ever said "out of character," or OOC. If you were to talk OOC, you would be called "insane" and talked to and possibly kicked out of the game. It was very much centered on role playing.
WoW is not anywhere near that strict, not even on the role play servers. It's pretty much policed by the players, and with the number of people in this world, it's impossible to have everyone always acting in character. But as a whole, I think our realm, Moon Guard, does a fine job of keeping things in the appropriate places.
Veritas, our guild, has the guild channel as an in character channel. We quantify this by saying we each carry a magical communication stone. A walkie talkie of sorts. To allow for other needs, we maintain an OOC chat channel to communicate with and help understand things in a more practical sense. The IC guild channel is a good idea because it allows us to role play more often than we would without it. Most of the time it's just hello or goodbye. The more creative will use it to request friends for dungeons or events.
It is discouraged to use the guild channel for personal role play stories and conversations. Basically, you shouldn't be forcing the whole guild to role play your characters situations by hogging the guild channel. Still, sometimes people use it in an interesting way to spark in person RP. Such as the time someone made garbled noises and and the sound of the stone dropping. Several people rushed to the last known location of that person, the plague lands, to see if they were okay.
As far as I'm concerned, anything and everything said with /say should be in character. If it were a real world, this would be plain talking, and ooc chatter has no place there. Whispers should probably be in character too, although surrounded by (()) brackets to indicate ooc talk seems to be acceptable. I tend to keep /party and /raid as ooc channels for the simple fact that I use those to communicate instructions to the instance and raid members. And figuring out how to do that in character is cumbersome.
I think I really need to work on my role play. I probably spend more time talking on ooc lines than I do ic lines, and it really wouldn't take much effort to do better on that.