Normal, attractive women (in general) will intuitively look first and foremost for signs of these two traits. Even if they don’t know it.

Strength is physical as well as mental, and strength is important for protection, for providing emotional stability and support during tough times for them especially when having offspring, important for his physical fitness and survival and ability to lift heavy things and working and constructing etc, and for being able to fight other men and assert power in order to assure survival and a safe living for her and their offspring.

Ability is likewise both a ‘physical’ trait (anything done with the hands and body) as well as a ‘mental’ trait (intelligence, knowledge, shrewdness, etc). Ability is important obviously when the man is the one who has to go out and provide and make a secure life (and the woman is the one who does the child thing).

Every little sign in a man that points to strength and ability will enhance his attractiveness more than anything else (along with symmetric facial features). The way he looks (both body and face), the way he talks, the way he acts. If any of these things show signs of strength and/or ability, then it attracts a woman.

Confidence, for example, is sexy bcuz it’s a sure sign of strength and also a sign that the man can go out and use and enhance his abilities. Confidence is therefore the perfect trait for a man, but the reason is bcuz it’s like the fuel needed to support and enhance his two most desirable traits. You cant be truly mentally strong without also being confident. Likewise, insecurity is a sure sign of the lack of strength.

Of course there are a host of variables which affect what a specific woman will find attractive, but for the healthy, attractive women in healthy societal settings this basic dynamic applies: strength and ability is what makes a man attractive. The more, the better.

Niceness? Doesn’t have anything to do with strength or ability, so for some women niceness is sexy, for others it’s not, like so many other things. As long as the man shows signs of strength and/or ability, he can be nice or not. Sometimes, for some women, in some settings, assholish behavior is sexy precisely as a sign of strength, showing that the man is mentally strong and will not be held back with his abilities. Reversely, if he is assholish in the wrong way it is a sign of the lack of mental strength and a sign that he cannot use his abilities bcuz he pushes away his surroundings.

Normally, niceness is a desirable trait precisely bcuz empathy is important for a man in so many ways, being a social creature, and the more happy he can make his surroundings, the more support he will have in all his endeavors and the stronger his social network will be. And then of course niceness in the wrong way is a sign of the lack of strength, a coping mechanism etc.

TL;DR: Everything (almost everything) that makes a man attractive to attractive women can be connected to these two fundamental traits: strength and ability. Or, a man not showing signs of these two traits will never be attractive to attractive women.