It seems to me from the show that characteristics of Jack which really stand out are his good heart and forgiving nature, his polyamorous tendencies, and a capacity to be protective and nurturing of those he cares about - characteristics which are seldom if ever played up in fanfic. It might be that other fans don't see him that way. It might be that they think it's sexier to emphasize other qualities. And in purely sexual terms, I think the most attractive qualities of Jack are that he is sensuous and tender - stories of him having rough or impersonal sex seem out of character to me. (But to each his own.)

He's fun to contemplate, in any case!