Good question! I can really only answer 'historically' at this point, since I haven't been playing lately.
There is rarely one thing. There are things that can tilt the scale in that direction, though -- and it's always one of them that's the straw that breaks the camel's back, barring over the top crazytown antics (which are thankfully somewhat rare).
Things that work against someone:
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Consistently disrespecting my time. This means not bothering to ever tell me if you're not going to show up, or making plans only to blow them off to go chase someone else.
Annoyingly, in my experience, this is often to go chase TS with someone -- confirmed, not guessed or speculated upon. I wish these people would just tell me 'I'm not interested in RP with you when there's available epoon to plow!' so I could go do something else and not set aside time for them. (Because I wouldn't.) -
Pressuring for OOC personal hookups/etc. This is a total no-go zone, and I'm up front about this. That... changes little, for the determined idiots.
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OOCly stalking me around the MUX. This could be +where-stalking, 'I see you are unfindable what are you up to?!' or the other variations on, 'how dare you be in a room with someone with a name that might be male!'
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As a sub-heading beneath the last bit, if someone has a bad thing to say about literally anyone I'm in a scene with that isn't them. This isn't always an accusation of something, and it usually takes a few weeks for this one to surface, since one instance of it is sometimes harmless.
There are less-than-awesome groups someone can fall into a scene with, and being warned that you've just slipped and fallen face-first into a toxic clique is not what I mean here. It's more the weeks-long progression of 'I see you're in a scene with Bob and Sandy, did you know that they totally cheated back on <game>? (Insert pile of gossip about Bob and Sandy here, repeat the next day when it's Tom, the next when it's Susan, etc.) This is a more insidious sort of 'I want to control who you play with' that's more subtle than the direct freakout over 'OMG YOU ARE IN A ROOM WITH BOB AND YOU ARE MAH PROPERTAY' but it is absolutely the same sort of bullshit control tactic with an extra dose of toxicity. -
Does not share attention well with others. Must always be the star/winner/focus of any scene they're in, no matter what it is or how many people are involved.
On the plus side:
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Creatively interactive. Picks up on hooks and throws them back. Does not always do the most predictable things (or the same thing on endless repeat).
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Sense of humor + does not always have to be perfect IC all the time. A sense of humor, and a sense of self-effacing humor (without getting into slapstick or self-pity) is a huge plus and adds a lot of fun to playing with someone for me.
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Shares attention well with others, and creates opportunities for others to have the spotlight.