I don't know if there is 1 thing any of us look for that make or break what entices us to set up an appointment. If I had to go with 1 thing, I would say the provider needs to be a safety girl. With that said here are the things I look for pretty much in order of importance for me:
1) Safety girl. I usually only set appointments if the provider reviews state that everything is covered. However, I have made exceptions but it's usually for an exceptional provider that meets all my other criteria. In the end the session turns into a safety session.
2) Hourglass figure. I don't look for really thin or really thick women. I look for the hourglass figure. I don't go for extremely thin or thick women but mostly in between.
3) Based on the TER rating system in reviews, I look for 7 or higher. A woman's body is more important for me than facial beauty, however, I do look for provider pictures that show their face.
4) Natural breasts, B cup or larger. I'm more of a size doesn't matter...a nice shape is more important than size. Bonus points if their nipples are puffy (very difficult to find).
5) Shapely legs and ass. I prefer a full bottom and thighs with calves that are shapely but not big.
6) I usually see providers in the 20-30 year old range. However, I have seen providers that are in their 50's. I think there is probably a larger number of providers in the 20-30 year range than over 30. Age is not a huge consideration for me as long as the other criteria are met.
7) Minimum of 3 reviews with at least a rating of 6 on performance. Due to criteria #1 above there is usually a top limit of 7 or 8 on performance since that usually means GFE.