Hmm I can think of a couple things that really make people stand out:
1. Enthusiasm. This is just true for mostly anything, right? I want to feel excited about a project, so it's definitely cool when a potential partner seems hyped about an idea too. We don't gotta be jumping up and down but a lukewarm "w/e" is boring, no?
2. Ideas. Other people have already mentioned this, but I like people who either contribute ideas or put spins on my ideas.
3. Writing sample. Don't make me dig, man. Presumably, we are all here to write with people whose style we vibe with. So like, make it easy for me!
4. Clarity. Tell me what you wanna write. Gimme an f-list or something to that effect. We don't gotta discuss the nitty gritty if you don't wanna, but trust that I read anything I'm given thoroughly. Stumbling around in the dark ain't fun outside of a horror game. Don't make me guess... as someone who primarily write dom, trying to 'guess' what is gucci and what is not is not as amusing as it might sound.