When I am a player in a RP, I want to feel like I have a purpose and a mission at the start. I don't want to just be let out into a world with the words "have fun!"
I like to have a clear goal or even an event to start an RP off with, as opposed to "here, fly free my pretties and try to make the best of it that you can".
Moreso than that, though, I think it's important to understand the setting of the RP before going in: particularly if it's set in a specific location. Sometimes an RP will tell you to go to Town A, without ever describing what Town A is like, and you end up having to sort of guess at how to describe your surroundings or inferring from other players; some of whom may describe the surroundings in a contradictory way.
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I think these two statements sum up my thoughts on the matter. I like having a clear mission/plot to start off with, lets me know I'm not let out on my own, I can plan with other people easier, and it makes it easier for me to come up with a post and not take a week to do it. Then if it turns more sandbox-y, I'm okay with that because I already have a bit of a start on my own.