For me, it's the smile.  A big smile gets me as hard as Chinese algebra*.

*A tip of the metaphoric hat to the inimitable Tom Waits for that one.

I can usually tell how things will go as soon as our eyes meet.

That she is not carrying.

That she does not have a look of panic or dread when seeing me.

That the closet door is open and there is not a pimp there.

That she does not have her Miranda card out and ready.

That she is not my wife.

That it is not my office throwing me a surprise party.

Not just the color and shape, but like your comment about her smile, a woman's eyes convey a ton of emotion.  Whether nervous, excited, passionate or bored, I think it comes across in her eyes more than anything.

The first thing I notice is her over all comfort level with me and our appointment.

I try to gauge her mind set and parallel / mirror her actions. I'm a regualar, clean cut, mid-forties, very polite experienced hobbyist; if I notice the lady is feeling uneasy about our appointment, I'm sure to slow things down a little until she settles in (usually a few minutes is all it takes). On the occasions where she is full game on right out of the gate and ready to rock, I go in that direction.

This hobby is all about two strangers having an intimate encounter, both the client and the provider need to be in sync for it to be enjoyable.

While her comfort level is important, if it's my first time with a gal the first thing I notice is how much she looks like her pix.  If she's not as pretty or if she's put on a lot of weight there's an immediate let-down.  It's hard for me to recover from that.  If, on the other hand, she's at least as good as her pix then it's off to the races.

I still channel Lt Colonel Slade (Al Pacino) when I catch the melody of a woman who wears her perfume well, "Now, that's a beautiful woman."

A smile overshadows all......similarly, on the other hand, the absence of a smile can be a game killer.

Another Tom Waits appreciator,

Giorgio

Add another vote for a warm, genuine, inviting and reassuring smile!

Her rack and her overall body. I don't look to make a connection in play for pay. I do it for the sex as she does it for the money. If guys want to go for all the personal connection stuff, I say have had it. I used to be more interested, but in the end, most if not all of the ladies have no emotional interest in me and I accept that. It's hard for lots of guys to accept it, but the ladies care about your ATM and if you want to test it, ask for credit.

What can I say I am a face man.

Strong perfumes turn me off, plus I am allergic to some and if I am allergic to her perfume I can't get close for heath reasons. Perfume is not cool in my book.

And when they don't laugh when I pull down my pants, oh yeah!

But seriously ladies and germs, a lady's eyes, and a wide smile melts my stone cold heart everytime!

A good nice genuine smile puts me at ease. I also like it if smells good be that a perfume, shampoo or scented shower gel but I think most pros don't wear any and use unscented shower gel so that you don't have to explain at home why do you have a strange smell on you.

It's amazing how some ladies just have this  sexy voice when you finally speak to them,  It just gets me going.  But once the door opens, it's the smile, then how they're dressed.

I love a pretty face and smile, but mostly how much makeup the provider is wearing.  I've seen some who had makeup caked on in multiple layers.  I was having lunch with a provider this summer. We were sitting outside and she obviously had lots of makeup on.  When she took her sunglasses off, there was two distinct spots on either side of her nose, marking where the legs of her sunglasses had sat.  I couldn't help picturing her without any makeup on and boy, the picture wasn't pretty.

gets on your clothing during the beginning of the date.  Yes, you can shower afterwards but what do you do if there is a perfume smell on your shirt or possibly lipstick on your collar?

sexy is she looking in her outfit; could be a dress, a robe, lingerie  or whatever.