Well rather than doing the polite thing, and reading the whole thread, I am going to just jump in
I try to always be kind. Which is not always the same thing as nice. I am not very personable, really. I am shy and don't say much to people unless I know them well. So I am not 'nice' in the "how is your Mom doing" sort of way. But if you have a toddler in your cart and you are behind me in line, there is a good chance I will tell you to cut in front of me. If I see you need help, I am likely to pitch in. I am the first person many of my friends call when they need help.
I am not nice in a fake way. There is a lady I am forced to work with fairly often. I think she is dishonest and mean, she is always putting others down and spreading rumors. I do not like her. I make no bones about it. I do not ask her how she is or inquire after her family. I do not even smile and say hello to her if I run into her out in the world. So maybe I am not nice. Not even polite, sometimes.
That said, I am kind to both people I know and strangers. I try not to gossip. I am not one that will say mean things behind your back. I do not spread rumors. I will always defend the underdog. I will not steal your parking spot, cut in front of you in line, or do anything else like that. So maybe I am nice.
I guess it all depends on how you define the word.
Oh, one other thing, I am very sarcastic. I also have a dry sense of humor. I know I have offended people by this, but I don't think it is a question of nice/not nice. Someone elses lack of humor is no reflection on me. Also in my family teasing is a way to show affection...