The first message is hella important. It is the opportunity to make a good impression. Always assume that you are competing with a couple others who would similarly be expressing interest as well.
To concisely summarise a few things that would sway (this is a personal point of view) my decision to take on a play:
1) OOC vibe. It matters a lot for me. I personally feel my best works are often with people whom I could talk thrash uncensored and just about anything under the sun that aren't play related. If two persons are on the same wavelength, you can literally write anything and trust your partners' intuition and thought processes when it comes to the play.
2) Plot muse. It fluctuates. This is ironic, but I am bored of most of my own ideas on my own RT. The PMs that really stood out for me are the ones who approached me by putting a spin to my existing ideas and turned them into something fresh. I adore novelties. If someone comes to me with a comprehensive and unique setup that challenges my own biases, it speaks volume about their creativity.
3) Long messages. I know quantity does not necessarily equates to quality. But I'm on the camp that believes that you can't have quality without quantity. I enjoy diving into the little details, nuances, thought process of characters and their history with great depth. You can't possibly do that with a few words. Once in a while, you might be able to capture all these essences concisely. But more often than not, concise is not sustainable if I am not looking for a quickie. One caveat is that obviously this defers between individuals. I know there are players here who would much rather play things casual and fuss free, or keep things old school like back in the messenger days where posts were a lot shorter and everything was quicker back and forth. But personally, I am more drawn to (first time) PMs that come with at or over 1k word count. If someone puts in that much effort into their message, I am more inclined to want to return the favour.
4) Writing samples. This will save everyone time and heartache.