First of all I think one of the biggest misconceptions is that we atheists are morally pliable because there are no strict set of guidelines for us. Although I subscribe to no particular rulebook my morals are firmly set. They started with the basic social norms as I think most do and have evolved over time through personal experience, introspection and empathy. I have critiqued them often and occasionally made an adjustment but they have remained pretty stable throughout my adult life. I do not claim moral perfection but I have a strong moral compass.
Another misconception is that our morality is unstable because we do not fear judgment. This is totally wrong, I know for a fact I will be judged based on my actions by a being that knows everything I do and even my intentions. As the arbiter of my moral code I am harshly critical of infractions and slow to forgive. I have neither a panacea of conscience nor an external scapegoat, my actions are entirely my own. If I harm another I must live with it knowing I have nobody to blame but myself. If I cannot correct the harm done I must live with the guilt which strengthens my resolve to do better in the future.
Next we have the idea that atheists have no motivation to good in the world. Although it is true I don't expect to be rewarded when my time here is over that in no way diminishes my capacity for altruism. The expectation of reward actually negates the possibility of true altruism. I do good in this world because I believe that selfless kindness is a virtue of the highest order and worthy of striving for. As Gandhi said, "be the change you wish to see in the world". I also believe that apathy is a form of immortality itself. Having a high capacity for empathy I cannot imagine ignoring suffering when it is within my power to ease it. I recognize my limitations in this regard but I do what I can.
Finally we come to the idea that an atheist's life is depressed and devoid of meaning. Again I can only speak for myself but my life is anything but depressed or meaningless. When I look at the world it is through my understanding of science. When I look at a living creature I see the incredible intricacies of its genetic structure and the symbiotic harmony of it's cellular structure. I see the 3.6 billion years of uninterrupted evolutionary struggle that crafted its form. I see the intergalactic journey of the atoms that compromise it and before that the atomic forge in the heart of a long dead star that created them. Is see all this and so much more and I am left in awe. Further amazed by the incredible chance to be the eyes by which the universe looks upon itself in wonder and filled with gratitude for this opportunity.