Good roleplaying is not in length, as many seem to think (though often times good roleplaying is found in length -- a good roleplayer often can write about simple things for a long time, and make it unboring). Good roleplaying is not in using four descriptive words for each object. It's not in using italics for thoughts or quotation marks around dialogue (though if you don't do that, your post will be pretty hard to read o.O) And good roleplaying does not involve rambling either.
Good roleplaying is in using the
right words. Good roleplaying is in fact in quality, not quantity (though I do say, most people 'round here lack both -.-

. Good roleplaying is in having an in depth character who actually makes his choices based on his character, not because the one roleplaying him thinks it would benefit him. Good roleplaying is in knowing that you can't (and shouldn't) win every fight, and in fact welcoming a loss or two every once in a while.
Good roleplaying comes in describing what your character sees. What he smells. What he feels. What he hears. Perhaps even what he taste. A good roleplayer should always strive to incorporate all five senses into his posts (though often, depending on what your roleplaying, this can be far from possible or quite illogical). Good roleplaying will tell you what the character is wearing, and this probably shouldn't be the same in every topic you're in.
Good roleplaying describes the landscape. It tells what plantation is around the character. It tells what animals are around the character. Good roleplaying describes the weather, and not just to the extent of it being cloudy or raining, but in depth -- describe the shape of a cloud as it passes by, or how the sun seems to be a red hot furnace beating down on you. That's a lot better than saying 'It was hot as he walked.' or even 'He stumbled along in the sweltering heat.'.
Good roleplaying will describe a characters thoughts. It will describe your characters emotions. It will describe why your character is where they are. It will describe where they are in the landscape. Good roleplaying will describe what posture your character is in. It describes what your character has with them. It describes where your character was before this, and where they plan to go.
A good roleplayer will have made a descriptive character profile that clearly describes their character. A good roleplayer can use this to give them inspiration when they have writer's block or similar diseases. While most roleplayers may prefer roleplaying in certain kinds of environments (usually a sort that matches their characters personality), a good roleplayer is not afraid to try something new -- and this will usually help quite a bit with having ideas.
Err... I could continue, but Snowy's bugging me to post in that 'A score to settle....' topic XD