Ok firstly a disclaimer. I don't know everything, I'm not a perfect RPer, what is expressed below are opinions and not divinely given truth.
Also what I determine RP quality by is the extent to which people want to read it, and want to keep reading more of it. This differs from a lot of other ideas on what makes RP good, so if you have a wildly different idea of what makes good RP you're probably going to disagree with several of the points outlined.
Uh, also TL;DR Warning.
Pacing.
People often seem unsure of how much description opposed to action or dialogue they should put in a post. In my opinion this boils down to an issue of pacing, dialogue reads very quickly, description reads very slowly by comparison. Good writing should combine elements of both to hit a natural feeling sweet spot somewhere in the middle there. If your writing reads too fast people will feel uncomfortable reading it, get lost and have to jump up a line or reread things sometimes, if it reads too slowly people will get bored and skim.
Different people have different preferences for pacing, but generally there is some consensus on a general zone of pacing which feels good to most people. It also means it's important to interleave sections of slow and fast writing. Don't put in huge blocks of description, or huge blocks of dialogue, but rather try to interweave them for a consistently paced sort of a mix.
Often good pacing requires sacrifice, shortening the description of something you really loved, or describing something you don't care about, or similarly cutting down on dialogue or action you like or adding some that's trivial. However annoying this might be the difference in how pleasant well paced writing is to read compared to badly paced writing is striking.
For more advanced writers it's also possible to change the pacing to create different feels or moods to the writing which might correspond to what's going on. For instance if I wanted to write a group of cheerleaders gossiping in my story I might use fast paced dialogue to convey a feeling of the conversation going quickly, people not taking a pause to breathe or thinking before they speak. i.e.
“Hey Mary did you hear about Pete?”
“That he got something from that skank he hooked up with?”
“Yeah, well I heard that it wasn't just from her.”
“He does get around, did you hear Sue was thinking about asking him out too?”
“No way!”
“Way.”
By contrast if I'm writing about a library I might slow the pace down to help convey the sombre feeling of the setting.
Bret Easton Ellis, the writer of American Psycho, uses slow pacing as well as a few other literary tricks in his books when describing highly violent or disturbing sections, using it as a tool to make the horrible feel mundane, and thus get the reader to identify with the perspective of the characters, to whom the horrible often is mundane.
Changing up the pacing is risky however, I've heard more than a few people describe Ellis' writing as too boring or slow for them.
Hooks
A hook is a dirty trick you play on your reader, where you don't give them all the information right away, but rather leave some things unsaid to lure them into reading more out of curiosity. For instance you might start a post with 'Bob looked fearfully at the dagger, knowing that he was about to die.' This hopefully prompts questions from the reader, why is someone pointing a dagger at him? Is he going to die? How will he get out of that situation? These will hook them in for the next few paragraphs at least, giving you the time to draw them in with your writing and latch onto them more firmly.
Hooks can come in many different forms, I forget all the names to be honest, but it's really all about grabbing someone's interest quickly and making them curious.
Doing a hook at the start of every post is pretty difficult, although it's probably a good idea to at least try to write them at the start of your first post in a thread if you can.
Show don't tell.
That should sound familiar to anyone who's ever taken a writing course. In your pose you shouldn't be telling me your character is an angry man, you should be showing me. Write about his angry thoughts, or better yet, have him act angrily, shove people, glare, say rude things to them etc.
In RP in particular, the order you should try to do things are; showing the reader with external actions/dialogue, showing with internal actions/dialogue, telling the reader.
The reason external actions/dialogue are of a higher priority than internal ones is because in a RP game you're interacting with other people, unless their characters are mind readers they can't interact with your thoughts at all, but should your character voice the thoughts or act upon them that creates things which add to the RP and that other people can do stuff with.
This connects with a sort of sub-point which is:
Don't just react.
Imagine this conversation.
"Hey Bob how're you?"
"I'm good."
"Cool, nice weather huh?"
"Yeah."
We can see how Bob is really giving the person talking to him nowhere to go with the conversation. As is true in conversation so also is true in RP. Your character should not simply respond to what other people are doing, but also should be doing things of their own for other people to respond to. Whether it's something as simple as asking other people questions in return, or something more complicated. It's generally good to check over posts to make sure that you've actually added something to the RP with them, beyond just responding to stuff.
Match Length to Content
There's an idea that often floats around RP games that longer posts are better. It's not really true at all, length is often a symptom of quality, it is however, never the cause. The longer your post the more time and energy you're asking from someone who's going to read it, it's a bigger investment on their part and they expect a better reward.
Which is to say if your post is twice as long as someone else's, it should have twice as much awesome goodness in it to be as good a read. Not a small ask.
Avoid feeling pressured to write a long pose just 'cause, or because someone else did it, and write what is fitting for the circumstance and reads the best. This connects a little to the point about pacing.
Long posts aren't bad, they can be great but only when people actually have a lot to write about. Don't be afraid of the shorter pose if that is what is best suited for what you have to say.
A well written long post is awesome, but a well written short post is better than a badly written long one.
Don't over-rely on character writing.
You love your character right? Of course you do, your character is your baby. But you don't want to be that person who brings everyone over to their house and shows them 250 photos of that time their baby got food on its face.
Similarly with writing, you have to realise that a lot of people will be reading your RP for the first time and don't know anything about your character yet and thus, don't really care about him or her yet. An important part of your job is to make them care, but in order for you to do that you have to get them to read your posts.
A lot of people fall into a trap of writing posts that are excessively character based, which is to say sections on describing the character's thoughts/feelings/history etc. dominating the post. This stuff is all important of course, but must be used in moderation. Remember many people, particularly those new to reading your writing, just aren't invested enough in your character yet to care much about all that stuff and going on and on about it is likely to make them lose interest.
It's very useful then to find a way to include things in your writing that will appeal to people regardless of how invested in your character they are at that time. There's two main ways to do this;
The first is to simply connect it to their character. Much like you love your char, they love theirs and thus if your post is connected to or important to their character it's pretty much a guarantee they'll care about it.
Obviously this is slightly limited however, so this is where the second thing comes into play. The second way is to find some content that strikes an emotional chord with the reader. If you go down the list of genres for fiction you'll see, romance, comedy, thriller, action, pornography, horror... All these are really about a particular emotional chord that they strike, being funny, or exciting or sweet etc.
In your writing you can try to use those same things, if you can make your writing funny, sexy, scary etc. people will want to read it so they can laugh or feel excited and so on.
Try to find which ones work for you and then put them in the post to draw people in and give them a reason to want to read your RP before they know your character, and as they start to read more and more of your RP they will come to love your character almost as much as you do quite by accident.
Addendum: I'm not saying put pornography in your posts btw. However there are ways to strike a 'sexy' emotional chord without being vulgar or distasteful if that's what sort of emotional chord you do want to strike.
Find a mean person.
Criticism is vital to improving your writing, unfortunately it's also very very hard to find. 95% of people when you ask them what did you think of this? Will say, 'It's great!' pretty much no matter how good it actually is. This is because people are nice. Try to find a less nice person who'll be honestly critical of your work.
It's also important when you do find them that you don't argue, when they say 'it seemed weird that he'd do this', don't tell them why it's not weird. Because the fact that they think there's a problem, means there is a problem.
Maybe the problem is that what he did was weird, maybe it's just that you haven't explained his motivations well enough but something in there isn't working.
Just smile, say thanks, go yell at a wall until you feel better then come back to it and work out what's not working.
If you can't find a mean person, try asking nice people in different ways. Instead of what did you think of this? Try, I think this is kinda boring or I think I didn't explain this very well, what did you think? Being more specific is usually a passable way to get more honest answers from nice people.
Err, the end.