(This may be a bit disjointed, as i'm kind of writing points as i think of them ( ; )
In all the posts i've ever been in, i've found that there are definitely similarities between the best threads and posters.
It seems they all have a good grounding in character. What i mean is that, no matter how odd the situation, or
taboo the subject, the characters always seemed grounded in reality. Even though the setting is completely fantasy, they all have a level of realism, whether it be in their dialogue, or in the way they act. It's usually why i put "-" and "..." in my writing, as i try to emulate the real movement and thinking in a conversation. Most of the people i threaded with before i took my time off had super good characters. Each character felt like they were walking in real shoes, rather than ones made up by someone else.
The problem with a lot of characters is that they react in ways no real person would. Of course, they don't have to be normal, but they have to make sense in the greater context of the character. If you're playing a passive, quiet traveller, who is skilled with a sword, and someone tries to fight them, if they cut their head off, they're going to be very shocked, and nauseous as they've just killed a man. They're not gonna brush it off, wipe the sword off their blade, then crack a joke, or say
"Shouldn't of ran at me... Kid.". They'd be visibly shaken. Some people forget their character, and write what seems cool and fitting, rather than what their character would. It doesn't have to make sense in the story, as long as it makes sense for the character.
Moreover, characters need to remember things. But it's a double edged sword; when you include something your character did three threads ago in a current one with different characters, you might seem jarring. But i'd do it anyway, as you're writing for yourself, more than for them.
I think a lesson loads of people need to take into consideration, especially new roleplayers, is that they should never forget who they are. When you thread, you are no longer Freddie Smith who lives in Watford England. You are
Maho Sparhawk, Hawk of the West and Champion of Imamu. If more people took that into their hearts, and remembered the context of their characters, not only would they be more believable, they'd grow along with the writer.
Also, just a peeve of mine i'd like to talk about, i typically don't think characters that are either A) super-old AND/OR B) super knowledgable are not only really hard to write, but kind of boring. When you write a character that has seen it all, done it all, and knows most-of-it-all, it's hard to believe that they're surprised by anything really. And if you plunge them into a mundane situation, it's hard to make conversation, and to believe that they're at all interested in what is going on. I think the best characters are vulnerable, but grounded. They are human; they have scars, faults, problems, but also qualities that make them good. That's why having positives and negatives are so vital to a character.
TL;DR: all the best threads and posts have characters that live and breathe. Characters that have lives, have done bad things, have seen bad things, and most importantly, have changed - for better or worse - because of them.