A good kisser (because I sure as hell don't kiss my friends), someone who I can see being a good father to my future kids, someone dependable to help me with bills and stuff in the future, someone I can see comfortably sharing a bed with, and I'm drawing a blank on a fifth one at the moment.
Here's the thing: friendship is platonic, and as a result, it can remove the potential for romantic feelings. So I would say that (often) things that friends lack in that regard is...
Intimate/sexual chemistry: The likelihood of me feeling that initial spark and sexual chemistry with a friend is much lower than that of a person who is not platonically linked to me who I am getting to know on a non-platonic level.
Mystery; getting to know the person can be a fun and deeply bonding process when taken out of a platonic context.
An unbiased perspective; they won't just comfort and agree with me because they're my friend, they'll give a more honest opinion about me and actually help me see flaws in myself in the beginning.
Excitement: it is really exciting getting to know somebody, let's be honest. It ignites all of these nerves and emotions.
Self improvement: Even if things fail with someone who has that unbiased opinion I mentioned, they kind of force you to look at yourself in a new light and make improvements to reduce the likelihood of failure in the future. Subsequently making you a better person.