Candor.
Of all Danny's qualities, this was among those I'm most jealous of. I can think of quite a few relationships, romantic and otherwise, that would have benefited from some more honesty.
I have to keep my jealousy in check, though, because we saw the result of too much candor in Danny's life. He never once hesitated or mitigated when it came to speaking his mind. It tended to alienate others, but I don't think that was because of what he said specifically so much as it was the fact that he had the gumption to speak candidly.
People like honestly only in moderation. This can only be chalked up as an inherent flaw, and in this sense Danny was miles ahead of us. His grand prize: an inability to interact with others as well as he may have wished.