On Niceness

"The nice guys are all over there. In seventh place."
~Leo Durocher
I've had trouble with this quote (and others like it) in the past -  I always considered myself to be a 'nice guy'. I was (and indeed am) friendly, courteous, and I like to think a little chivalrous. Did that mean I was destined to be a failure, to finish in seventh?
I came to a deeper understanding of this not long ago, mainly owing to a childhood memory. One of my primary school teachers (I cannot remember which one, I'm afraid - all those years seem to blend into one in my mind) told us to never write the word 'nice' in a book report, because it's such a vague, nondescript word.
I think that's the point here - 'nice' does not simply mean chivalrous, courteous, and all those things above, but nondescript, satisfactory, agreeable. In a word, mediocre.

I started this post with a quote and I will end it with another quote:
"Aim above morality. Be not simply good, be good for something."
~Henry David Thoreau