1. Intelligence: a gentleman should be well-read in multiple fields of literature, able to competently discourse about the news of his time, and possess an intellectual curiosity that respects the beauty of interdisciplinary thought. As a corollary, he should also be versed in language(s) to such fluency as will allow him to express his thoughts succinctly and elegantly.
2. Humor: every gentleman should have in his repertoire the spear of wit, the club of pun, and the blade of irony–for what is the meaning of a life without laughter?
3. Humility: despite striving for excellence in all he does, a gentleman will never show arrogance. He will never assume the cloak of superiority above another, instead accepting that he will always have much to learn. Vaunts are unacceptable and unnecessary, as prowess married to finesse has a voice of its own.
4. Compassion: a gentleman should be sympathetic and empathetic to the plights of others, and possess the refined sensitivity to detect discomfort even when not explicitly expressed.
5. Generosity: a gentleman should be giving of his time and efforts to others. He should be quick to offer aid of all forms without prompt, although in balance he should possess the faculties to carefully judge priorities so as not to commit beyond his means.
6. Punctuality: a gentleman is always early unless courted by such overwhelming forces as could not be foreseen and accounted for–for instance, serving as sole witness to an accident while en route to a previously planned rendezvous.
7. Courage: throughout his life, a gentleman will cultivate aptitude both physical and mental such that he may possess the daring to face challenges and tackle obstacles with confidence.
8. Wisdom: while a gentleman should be passionate, he should possess a temperament cool enough to realize there are occasionally limits, as well as boundaries that should not be crossed. He should understand that anger solves nothing, that recklessness leads to demise, that excess leads to greedy sorrow, and that human bonds should be treasured above all else.
9. Aura: the most subtle of qualities, and intangible*–a true gentleman has about him an inexplicable air of quiet strength, a chivalrous feel that commands and gives respect, and an undeniable sense that there is far more than meets the eye.
10. Love: I have saved the best attribute for last–a gentleman loves his family, his friends, and life. This love is the driving force of all his other qualities, and he will defend it to his dying day.

Disclaimer: I do not consider this list definitive, and as such I welcome suggestions!

* While many have considered visible groom and polish to be defining features of a gentleman, I cannot in good faith list it here after seeing an unshaven and unclean homeless man rush to hold a door open for a young woman exiting a store, her hands full with the spoils of her shopping endeavors. While it was in broad daylight, I initially suspected him to intend harm and began sprinting in their direction, only to be quickly ashamed of my assumption as he bowed and even held his arm out in flair as the young woman thanked him warmly and continued about her way.