i think it depends on your perspective and your intended audience.

in general and overall, i completely agree with bagginsbill's statements when it comes to materials and light.  i highly recommend exploring the capabilities of Poser as an application.

i've seen images here that made everyone "oooh" and "ahhh" and made my boyfriend exclaim at how horrible they looked when he saw them over my shoulder (one office).  why?  because the Poser community is extremely tolerant of anatomical and posing problems with figures and clothes to the point of blindness.  myself included.  and still, i can see that each of those images, except "Jordan," shows a pretty good example of V4's anatomical flaws.  and they're not very dynamic poses, so they don't show some of the more extreme anatomical problems or hair and clothing problems.  if you notice, "Jordan" avoids a lot of the typical Poser figure problems by not showing anything more than face, being a custom morph, and having postworked hair.  and not showing enough of the sweater to show any of the typical "floating" problems conforming clothes (the most common type) have. 

if your audience is CG people outside of Poser (who are sticklers for anatomy)  or even your average citizen who isn't almost instinctively blind to various errors (primarily those of the reigning Vicky), then there's only two solutions i know of for realism.  lots of custom morphs specific to your figure's pose or postwork in Photoshop.  major problem areas are eyes (especially the eyelids), shoulders, elbows, knees, ankles, feet and hands.  hands and feet are so much of a problem that i can usually use them to spot the DAZ figures within completely repainted works on CG Society. 

if you want to do really dynamic works, Photoshop, another image editor, or  a really good custom morph tool will be absolutely necessary for realism.    even more importantly, spend some time drawing or painting from life or photos so you know how something should look.  and what proportions a figure should have.  sure, there's a debatable range, but when your figure's elbow is half a head length above her head when she raises it, you just need to do something about it. 

oh!  and expressions might be an issue.  also, textures stretch and distort in certain places when figures are in certain poses.

and remember that what people think is there is usually not what is there.  a lot of learning to draw realistically is learning to draw only what you actually see rather than what you expect.  the same rule applies to 3d if you are concerned with figure realism.

if you're looking for really, really high end realistic lighting effects, like caustics (example: light patterns beneath a crystal goblet), colored shadows (examples:  light through stained glass, the glasses shadows wine or juice cast), or even just true global illumination  (example: a nude whose arm redder on the inside due to reflected light from the body), then you need a high end renderer more than you need Photoshop.   well, you can kind of do the last one in post, but the rest are more difficult.  so if you wanted to focus on very specific types of materials with a CG professional level of accuracy, you should probably look at rendering with something besides Poser.  there are free renderers  out there, and ways to accomplish this.  but i'd say, mostly, people don't notice when those are absent, and for the types of renders 99.9% of the people do around here, most of those effects are unnecessary. 

if stained glass were a requirement of that temple vicky's always strolling around naked with her sword, it would be another story.  and if you want to do mainly still lives and/or explore effects like those, you might want another tool.

effects like depth or motion blur are mostly more effectively and quickly done with an image editor and a depth map render. 

i have personally run into walls with shadows in certain lighting situations.  the most significant recently involved a light at a certain angle and a plane with cloth dynamics and medium heavy displacement.  i tried and tried to get raytraced shadows to work for that image.  raytraced shadows and blur with distance as real shadows do (though i'm not sure they do it at a rate that's technically accurate). they just didn't work.  i had to switch to depth mapped shadows.  but i've had depth mapped shadows just disappear in certain spots with fine objects (jewelry, for instance).  so i've had to postwork shadows in certain situations.

a lot of the postwork in my gallery is obvious, because i like to paint hair and i like to play in Photoshop.  but in any one of those there's a lot of corrections i made that aren't so easy to detect and just get rid of problems. 

oh! and a technique for maximum realism and effect is to render each aspect separately then combine them in an image editor.  one render for just diffuse colors, one render for shadows, one for specular, etc.