My preferences on length and G/S competence actually vary based on the type of roleplay I'm currently doing. Something that's more smut/sex oriented, ones that are more there to satisfy a kink/scratch an itch, I don't mind shorter roleplays, shorter posts, and as long as spelling and grammar are reasonable, I'll be fine. As I'm not being pulled out of verisimilitude (not sure if that phrasing is exactly correct but w/e) every other sentence because it takes me a moment to decipher what is being written. Spelling errors are fine as long as they aren't gratuitous (I often google a word when I think I'm way off on spelling) and precise grammar isn't important as long as I can understand what you meant. No "txt speech" (u, 2, r, h8, etc.) please. (the obvious exception being if the characters are actually texting or on an online chat and they would communicate like that).
However, in the case of threads that are more serious in intent, ones that I'm focused on the plot or characters and not just the acts or simple scenario, I tend to prefer Long (1-3 paragraphs) or longer posts on average. I don't mind posts being a bit short if you don't have much to say at that particular moment but want the thread to move along. However, I'd rather a thread not devolve into 2-4 sentences per post. I don't mind waiting on a reply for something with more substance, or just reaching out and saying "I don't have anything to say here" and moving along. For these threads, I prefer grammar and spelling to be knowledgeable. I'm not going to get overly academic - I won't be critical of the excessive use of commas or get upset about split infinitives - but I treat these sorts of roleplaying as collaborative storytelling, and as such would prefer to maintain that sort of consistent style of fiction. I have a strong preference for third person perspective.
I'm also not a huge fan of Japanese culture. I don't mind if a character is intentionally Japanese for story purposes, but my default assumption would be that the threads are taking place somewhere more similar to the US (or whatever fantasy trope). For the most part, this isn't a big thing, I don't mind taking parts from that country/culture that you might find interesting, but I will say this specifically: I do not like the use of Japanese honorifics.