I agree. It's a fine line being professional and showing you affection at the same time. if it's reached in this narrow window it's bliss. It is extremely rare and almost impossible to find. Personality is what I put, but as others have echoed it's a combination of everything.
I would rate in order of decreasing importance:
1) personality match. Most are looking for an escape and to unwind and destress. The most annoying thing is spending time paying for someone you don't want to be around. A shared sense of humour goes a long way along with same tastes in music, film, and other arts. There is nothing more sexy than a woman who knows how to politely banter away with you and debate all manner of topics. A similar level of educational background helps.
2) affection or illusion of affection. Obviously I kind of enjoy it if they really remember what was discussed at the previous session. It personalizes the experience in what is not really a personal service. It also establishes intimacy. I was most amazed and touched by the escorts who remembered everything from the previous session. These ladies have definitely distinguished themselves.
3) professionalism, service quality and consistency. I am going to take some heat for this but I honestly prefer girls advertised and reviewed as safe only if I am getting Gfe pse. I think reliability goes in here too. If I am getting a lot more mileage it's a privilege and I know that there is at least an element of our encounter that is special. Reliability I'd lump in here too. My time is valuable and they should respect my time, especially given that I am the customer.
4) sexy flirty communications that is genuine or at least comes off that way. Ok, I'll admit it, most of the time I am bored at work and it's a breath of fresh air when an escort emails me out of the blue letting me know she's thinking about me and wondering when were going to meet next. Flirty emails, tweets, or other forms of discrete contact. Saying she's fantasizing about our next meeting out of the blue. I know on a few occasions these unsolicited contacts were motivated by more than "marketing" and came from genuine interest. I've always looked for almost a mistress like relationship so this kind of stuff makes me smile.
5) looks. As long as they are attractive I think it's fine. I see a few ladies that are 10/10 but my ATF was an 7.5-8/10 for the aforementioned reasons. She was the whole package.
6) price. Luckily no escort is beyond my price range but like any guy I prefer value. In this industry price does not in any way correlate with service, looks, or reliability.
It's entirely dependent on intangibles like the above which can be summarized with "chemistry". keep it light, flirty, and just a fun fantasy.
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My advice to sps: the trick is make it seem like real affection without letting it become real affection for either party.