Women generally respond to power and men who wield it, in its many forms. This power can be several different things including the following:
Looks, as you suggested. This can always get women sexually interested.
Money, a pretty obvious one and a way where ugly guys frequently end up with good looking women. By being with men with money, the women can overlook the visual completely, since many other needs are satisfied with financial stability that a wealthy man may have.
Job status, where if a guy is in a higher position in a company or industry, a woman could take interest in them, knowing the man can help elevate her in her own career.
Humor. Women respond a lot to humor and ugly guys who have good humor may also have pretty good personalities. The humor may get the woman initially interested and if there is a good personality behind it, it certainly can be good enough for them to overlook the visual.
Psychological power. Many guys treat women terribly, yet the women stay with them. The women are subconsciously responding to the fact that they generally respond to a man who is in control and who takes charge. A lot of times guys play up on the insecurities women feel and exert a control over them. Even ugly guys frequently are able to do this.
Having self confidence is the most important thing for any guy to attract women, though.