Can be boiled down to a few things really, if a partner has all of these, then I consider them a good partner.
Communication: It's already been said so let's get it out of the way, this one is important. Tell me when something is wrong, tell me if there is something you'd like in the story, tell me if I need to change something or if I made a mistake. Talk to me, it helps!
Patience: Sometimes I will get your reply to you fast, sometimes I will be busy even if I'm on the site. Making me feel that I have to reply now is no good, won't serve either of us. Have patience with me and I'll have patience with you and we will be just fine.
Creativity: Nothing can feel more exhausting than the feeling you're the only one driving the plot and/or coming up with ideas. If you have nothing to add and are just along for the ride I'm sure it can feel fine but it can be quite draining. I'm sure some don't mind, but I'm here to collaborate, I want and need your input for this to work.
Don't be a cunt: I dislike rude people. You don't have to be super chatty, it's nice but not a requirement for us to write well together, but if you're snarky and passive-aggressive then I'll be ending it rather quickly. This category holds a special award for those that try to jarringly pull the RP in a different direction from where it is going because they want something that isn't happening naturally, try to manipulate your character into a situation you clearly don't want them in or ignores what you write if it isn't exactly what they wanted to read. It's a collaboration, we write together, I am not there to service your RP.