I completely agree with Anton on this.  I hear all the time how Poser "can't" be realistic, because they're not advanced enough.  I look at it in this sense:
What is the difference between an excellent photograph and a snapshot?
Most of the things that make the difference, Anton's already listed, and I won't do it again, because, repetition sometimes sucks.  You have to know how to use the cameras and the settings in the cameras, as well as how to use the lighting.  Textures, figures, all that stuff isn't as important for realism as making sure you know what to do with your camera settings and lighting.  If you look at the work in the galleries, the most realistic looking work, the artists have an honest command of their lighting and camera settings.  The render engine CAN do wonders, but you have to actually work at it, not do a point, click, and hope.  :D  You can use the most hyper-real textures, use photographic backgrounds, merge photographic hair onto your figure, and it can still come out looking fake, even hokey. 
In the end, it takes skill, it takes practice, and very few of the members here have mastered it yet.  It's wonderful to see when you come across it, and it takes a looooooooong time to learn.  Most of us are still in the early stages of learning realism.

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Knowledge is knowing a tomato is a fruit. Wisdom is not putting it into a fruit salad.