Writer's block. Every writer's personal archenemy. It's destroyed me so many times, but I found a way to break out of it, so (fingers crossed!) there will be less instances of letting people know 'Hey, I'm so sorry, I've been plagued with the inability to write - again!'
Overthinking. Especially when I find people who are 100000000 leagues better than me at writing. I worry so much that my contribution will turn them away from me! XD So then I panic and take an unnecessarily long time to reply.
Sentence structure(? Not sure if that's the right term). I heard once that varied sentence length has a nice, cool flow. And then there's me. For some odd reason, I like my sentences broken by at least one comma. So I have to write my thoughts out, then see if I can whittle down sentences to make things flow better.
Keeping very interactive dialogue going. Don't get me wrong, I can do it. Dialogue is one of my better skills, but for whatever reason, my ideas for questions burn out. Or maybe it's just when I suddenly find two characters only asking questions. I dunno, I'm looking back at one specific RP, but now that I think about it, it's because my partner insisted on their character being locked in a brig, meaning there was only so much that could happen between the characters. Aaaannnd now I'm kind of unsure of my point. XD Just word of advice! Don't expect me to keep peppering your characters with questions, because I can't do it for extended periods of time!
Also, sass. I'm too nice in real life, and don't have a sass bone in my body unless I get very tired. So I struggle with sass and quips and roasts and whatnot towards other characters. BUT! I do get some rare inspiration and tend to slam characters when I do see an opening. But I can't seem to keep witty banter up for a long time, lord help me. I'm working on it - and please, if you write with me, you are more than welcome to tease me about my horrible attempt to burn another character. XD I take teasing a little too well.