Wit and intelligence.
Most women are interesting in bed, in a delightful if limited way.
Is she interesting out of bed? Funny? Intellectually stimulating? Do you have similar, or at least compatible, interests and goals?
For a long term thing you'll be spending a lot more time doing things besides bedroom activities. Is she the one you want to do that stuff with?
If you want to deal with her outside of sexual contact, that is a good indicator.
Speaking for myself. A woman can get you in two ways, by your heart or by another part of your body. If she gets you by your heart you feel like putting your arm around her shoulder. You feel like taking your coat off and putting over her in the rain. You feel like you want to walk with her by your side along the beach. You feel very protective and caring towards her.
If she gets you by the other part, you just want to get her panties off and have a real good time.
1) Mental Stability: My mom has had enough of my crazy girlfriends
2) Responsibility: If I'm the only one setting dates or making time to spend together, that may be more romantic, but it's not long term
3) Their Comfort Level: The right girl just "fits" and if we have similar needs as far as the amount of affection in our romantic lives and the amount of privacy in our personal lives, that's what's going to make the relationship seem "effortless."
If it just you two or just your friends it a fling . If your introduced to his friends and associates your a keeper .