Things that make me want to RP more with a person: their RP is fun and they are nice OOC. RP being fun can be pretty subjective, but -- picking up what I'm dropping, giving stuff in return, etc. Being attentive and responsive. Having a sense of specificity to their PC, which is what makes PCs really seem alive to me instead of kind of generic. Being generally considerate and a good sport.
Things that turn me off? In actual RP terms, I really hate a sense of IC gaslighting. Even when it's totally legit IC, it just gets under my skin and I don't like playing with it, so I'm likely to avoid that character or player in the future. By this I mean that sense of, like -- PC A does something super rude or whatever, my PC reacts like it's rude, and then I get a whole bunch of "omg you're so unreasonable/I wasn't being rude" back IC. I think this is more likely just a level of OOC unawareness, but it super bugs me, so I avoid it when I encounter it.
OOC clingyness, rudeness, etc. I don't have a problem with RPing with a person a bunch if we're both really feeling it, so long as we're both also chill with the other being like "Okay gonna go RP with some other folks!" I've had RP partners like that and it's totally great.
I'll say disrespecting my time rather than wasting. If we have plans and RL happens, it happens! But please don't then leave it on me to follow up when you come back around the game. If I get suddenly held up and miss someone, I'll be paging or mailing them as soon as I'm back to the game for a quick apology. The whole "miss a planned thing and then don't even bother to say anything" drives me a bit nuts.
I tend to be on channels a lot, so just -- idk man, sometimes I find someone generally aggravating in how they talk on channel, and then I don't particularly want to RP with them. If you are OOCly rude, patronizing, etc., where I can see, I probably don't want to play with you.
Being demanding/entitled about RP or in RP. Being inconsiderate.
I WILL ADD MORE IF I THINK OF MORE.