I worry about writing quality generally when I'm gming an rp. I worry that I'm not setting the right tone or mood for the scene. I wonder if I put enough descriptions for players to get a sense of the setting. If the idea I have is even good enough to bother writing down. This often leads me to take forever writing a post when I'm the gm, which leads to a chain of procrastinations.
Otherwise, I don't think I worry too much about what others think of my writing. When I join another's rp, I write for myself and my characters. Sometimes I may take a while, nitpick certain things, but I attribute that to mild perfectionist tendencies and undiagnosed add, which makes it difficult to organize my thoughts into words the way I want them to. I can sometimes take hours writing a post, re-writing lines, refusing to post my writing until I'm satisfied, but that has less to do what others think of my post and more to do with getting everything that's in my head down in written format so it flows nicely and doesn't sound too overbearing given the mess of idea and thoughts in there. I probably juggle 3+ possible responses to a single post in my head, weighing things like my character's personality, what they would do in that particular situation, whether said action would help or hinder the rp, etc, etc.
But otherwise, I don't feel intimidated writing with people whose writing is clearly superior to mine. I write as much as my muse inspires me. My goal in rping/writing has more to do with progression than writing the best novel about my character and, though I do rp for them, it's more to see their progression in the rp than anything else. Hence why the actual "quality" of my writing is secondary to me. I mean, I do go out of my way to make my post readable, and will get frustrated when it doesn't flow as well as it did in my head, but I'm not a grammar nazi when it comes to my post if that makes any sense =p I'll correct any errors/typos I see, but that's because it bothers my eyes and not because I expect people to read it and care about my little mistakes. People can think what they want, but as long as I'm satisfied with my post I'm generally content.
Of course, that's only when I'm not gming <.< When I'm in charge of an rp, I write for other people rather than myself, for the plot rather than the character, and it gets a bit nerve-wracking.