Here's what I know...
(1) The ability to resolve conflict is key. You need to be able to discuss a circumstance with a person and work through it quickly, openly and move forward.
(2) You need to laugh at the same jokes and find the same things funny.
(3) The kiss is key. If your kiss is good, if you fell into a good kiss right away or were able to refine the kiss immediately and you can kiss for hours and send little tingles down the other person's spine with your kiss, then you are golden. Good kissing foreshadows good sex.
(4) Time needs to go by effortlessly, quickly and without noticing whenever you speak or see each other.
(5) You need to speak a silent language- this comes from having a similar sense of humor.
(6) You need to both be able to be self deprecating- give it as well as you can take it and be good-natured in that area.
(7) You need to trust each other and feel comfortable that the other one has your back and will always have your back.