Green flags:
Metaconversational skills! Introductions and getting to know can be super stressful, presenting awareness for this process and being aware of how difficult communication is, being able to shift to a meta level and actively help building a forthcoming exchange is a huge thing.
A good grip on realities! Being able to go deep into fantasizing mode, while understanding what it is and relates to what we do in real life matters so much.
Focus! Especially in online interactions, if someone is focusing on the conversation (responding quickly and thoughtfully) is a huge plus. I do not expect this every time but when I set out to have a conversation with someone I put my focus on them and don't do ten other things at the same time.
Red flags:
Single-mindedness. Looking for just one very specific thing? Expecting conversation to go a particular way? Not even acknowledging anything other than what you have in your mind? Bye.
Impatience. Insisting on playing right away? Not taking no for an answer? Not with me.
Manipulation/misleading. A potential sub trying to sugarcoat their limits to make them seem more attractive? Bragging about or exaggerating experience? Not going to fly with me.
Attitude. Jumping into submissive or dominant demeanor without prior agreement? Taking yourself or the subject too seriously? Making assumptions and going with them? Eww.