This article is a work-in-progress, and is certainly not meant to be all-encompassing. These are just a few of my own personal opinions on what constitutes good (or bad) roleplaying.
All this is to emphasize that the online environment is much more immersive than the tabletop venue. And as such, players can stay "in character" (IC) much more easily, and many players prefer to. Some extreme players never go "out of character" (OOC). Respect this. If you're with a group that is not chattering in OOC mode, please don't assume you doing so is okay.
Don't Make Others Wait. Here, I'm referring to typing speed. When you're engaged in dialogue with others, every moment counts, in order to maintain the flow and make it seem like a real conversation. Have a lot to say, but don't type quickly? Then don't type a huge paragraph. Type a sentence, post it, then continue. Others will thank you. Or at least not silently curse you.
Be Here Now. And by this, I mean that you should remember that your roleplaying is happening in the present, not the past. So when you do an emote, do it in the present tense, not the past. For example, "/em laughed at the joke" should be "laughs at the joke." Many role players imagine that they are telling a story. And yes, you are. But you're doing it as an actor on stage, not as the writer of the script. Right here. Right now.
We're Not Mind-Readers. Okay, some of us might be. What I'm referring to is the tendency for some roleplayers to type in their emotes what they are thinking. But folks, you're not writing a story. You're acting. Other players/characters don't have a clue what you're thinking, any more than the audience does at a play. Use emotes as an actor does, with facial expressions and other non-verbal cues.
Don't Play God. Simply put: you control your character; you don't control others. You cannot tell other players what their characters do, how their characters react, etc. And this goes for how your character, and his/her ability, interacts with others. For example, you may be playing a character with extremely strong telepathic powers, but this does not mean that you can read every other character's mind or control their actions willy-nilly. That's not good gaming conduct, for one thing, but also is very presumptous. To create a character who is "the most powerful" or "immune to all" is known as "godmodding." In tabletop gaming, this is often done by maxing out all attributes, choosing the most powerful combinations of spells, etc. Basically, doing everything you can to make your character unbeatable. While this sounds attractive, it's also boring. Such characters are invariably superficial and uninteresting, and usually annoying to more serious gamers. Interesting characters are flawed characters. No one wants to hear of the exploits of someone who can't be harmed, for whom nothing is a challenge. Talk about a yawnfest. Bring your characters down to earth.
Magic. This is quite possibly the most frequently abused plot device. Yes, magic certainly has a place in online roleplaying, but it must have limits. To be able to "magic away" any possible threats or challenges is a huge copout. An example: a character has contracted a potentially fatal disease. Another character says, "I can cure you with my magic." No. That's a copout. And also godmodding (see above). You can be a powerful mage, sure. But the best you should do is say, "I can attempt to heal you with my magic." Remember, it's up to the player of the afflicted character to decide if your attempt works.
Aliens. Arthur C. Clarke said that "any sufficiently advanced technology is indistinguishable from magic." And that's true... to the less advanced societies, though not to those actually developing the technology. Aliens and their tech have the same potential for abuse. To use the same scenario as above, your teammate-from-another-planet says, "I will go meet with my people, the Clarkeans. Our medical technology will save you." You get the idea.
Time Travel. Talk about a can of worms. The appeal of time travel stories is huge, of course, but consider the slippery slope you create by introducing it. "Cancer? I'll pop to the future and obtain a medicine to cure you. BRB!" Or, "I'll jaunt to the past and prevent you from getting that horrible disease from that neutronium leak!"
Of course, deus ex machina doesn't have to be anything so grandiose. It can be fairly mundane, but is still a contrived, unnecessary "fix" to a problem, one that fails to challenge a character. Simply put: don't go there.