While I haven't RPed in a while, some things that have always been a massive issue for me are these.
CSS - While I don't mind and I know they aren't required. When you don't have one, it feels like you are writing less than you actually are. And when you pose without one, it stretches a line of words to around 25 or more words depending on the website layout you use.
Amount of people - I don't mind having a lot of people in say Pokemon Trainer Academy RP from way back when. You didn't have to worry about traveling in large groups and you couldn't really get behind because classes didn't matter too much. They were just their to add to the RP. But when there is.... let's say more than 7 in a travel based rp, that gets hard to catch up.
Story - If I am honest, the story is almost always the same for pokemon. There usually isn't many unique ones other than one off events that we have which are small. While I like sandbox, I feel like a story should be present. While I do like a journey rp, why does it have to be secluded to a single region? Make arcs where you can go between regions and explore more. And while we are using old regions, make some unique places to visit.
Idea of not finishing - And might just be me, but I just know at some point the RP is going to die. I don't want to join an rp when it is going to die within 3 to 4 months of it going on.
Post Restriction - While I understand this is mainly so people don't get behind, when everyone ends up waiting for a single person ,who is slower at posting because of a post order, it kills the momentum of the RP and people begin to fall off.
And I know this last ones are very controversial and it is different for other people, but....
Elitism - I know people don't expect everyone to be perfect. But there is a such thing as being too picky when you are DM/GMing. You have to allow characters to grow. If they don't have a fear yet, then it is the DM/GM's job to throw something at them during their posts. If there fear is water.... then just let it be water.... maybe if there isn't a reason yet, then perhaps they can come up and reveal it as they go through. If someone doesn't have a goal yet, why force them to make one? Maybe they haven't encountered someone in their life to give them a goal. If someone wants to catch a pokemon, then let them catch it.... Obviously you can't god mod and catch everything, but if you have to do a good battle scene every time you want a pokemon, it is going to get old fast. Let the players have a bit more freedom. I know a lot of rps do give people freedom, but sometimes and just feels like gate keeping.
Realism - This is for fantasy and pokemon mostly, but while I like realism... you can't be 100% realistic going by our world. The characters live in a world of creatures who can do all sorts of things... or is a magic world where most logic is non existent. While there should be some realism, you can't be too realistic in a fantasy setting.
Oh one more.
Entry Length - I don't think a single post should take more than 2 posts each. Just save some for later. When someone posts 2 or more pages and it is just that character, it makes people feel alienated who can't write that much in a single post. A JP is different, especially if there is more than a few people. I feel like that this, and I've done it before when I first began RPing, is a way to stop people from interacting with your character. It also just makes people not want to read three pages worth of writing. So if they are interacting with you, they might miss details. I mean I've had trouble rping with people in video games because they don't want to read 3 to 5 sentences and completely miss stuff.