Yeah you're paying a monthly feee for content the creators choose to upload. There's a comment section for each piece of content (video, photo) uploaded. You could leave suggestions there per se, but in most cases that comes off as a subscriber complaining.
My only issue with OF and J4F is there's no preview of the content. So you're buying into something blindly. Some guys are great at posting previews on their Twitter accounts (Austin Wolf, BigCMen/Jared Grey) but most times it's a guessing game.