I'm normally polite--let people merge in traffic unless I think they are taking advantage of the situation (cutting in front of people), don't cut people off when walking in the grocery store, will help someone who seems to need help, etc. So yes, I'm nice.
However, I will not allow myself to be walked all over, which I think for some people is their definition of nice. Some people don't like any confrontation and won't speak up about anything and they call that nice. I try to speak up politely, at least at first, but I will speak up. I try to pick and choose the battles I fight, which has changed some throughout the years.
Being polite and respectful doesn't mean always being nice and it doesn't mean being a doormat.