Something to keep in mind is if you're new to Mushes and you tell people you're new to Mushes, people will read into that. And many will try to be helpful (or busybody or plain manipulative) and tell you what is right and wrong, but that will most likely be based on their personal preferences and then inflated into ubiquity. As anyone who has traveled to different countries might tell you, you'll get as much failadvise as real advise.
I'm now on my 5th year of Mushing, and damn time has flown. I've never been on a Mud, but I have roleplayed on different mediums that have put different weight on what I think are the two main axes: Are you playing the Game or are you writing a satisfying Story (a very unfocused, bad, and even when its not horrible still in badly need of massive editing, story)?
Different Mushes put different emphasis of what should matter most, thus growing different cultures in their playerbases over time, then die and those players go out in the world and spread those culture spores to new places. Clashes of culture and expectation happen, leaving a hodge-podge in its wake. I can tell you that in 5 years I've still not come across a whole lot of hard rules as to what's okay or not, only a lot of often-conflicting personal opinions. Some exceptions are:
Malicious or ridiculing metaposing will get people bitching like crazy, because its taken (and often meant to be) a passive-aggressive critique of characters and players they feel helpless to respond to. Conversely because a lot of Mushe(r)(s) weigh heavily towards story/character development they're happy to metapose quite a lot to fill in that story as the play happens. A personal favorite of mine is self ironic meta posing that pokes fun at one's own character.
Entering private grid spaces uninvited. While some people will frown at you for crashing any public space too, I say fuck them and do it without blinking an eye. Also there's +hangouts on most games to add the facilitating of spontaneous rp.
Personally I really dislike too much OOC communication, especially when it (and it often does) lends itself towards OOC manipulation of events and layering pressures and expectations of what should happen, for fishing for ways to avoid even slightly unfavorable consequences. But a lot of players have a vastly different opinion of me on it, believing OOC communication to be the key to happy funtimes. My solution has just been to do it my way, deal with the occasional (and there's never been much of it) fallout with a shrug and a smile. (ETA: Or a cyber screaming match. WHATEVER WORKS.)
