Great post! Thanks.
I feel that the way I stay nice but not too nice is to be very OPEN. In most of the situations above, I will try to say the truth even if it may hurt some colleague's feeling. I always feel that it's better for long run to gentally let a friend know her/his problem instead of fixing the problem for her/him. It may take a lot of time to communicate and wait, or even sometimes it hurts the relationship, but after a while, people will know your good intention.