If you have the .psd you can probably make a run at fixing it. I have no real idea what's causing it, but nothing is permanent if you've been using your layers correctly.
I've run into people here that have freaked out big time if I've given them any kind of constructive criticism, so I try to be a little bit more cautious.
Ahh. Do you go to Save for Web, and the optimize the compression to keep it as high quality as possible? Just checking, because it is something that is sometimes over looked.
As for the difference here's a small test for you:
If you cannot tell the difference between these two images, then I would be highly surprised. It's essentially the difference you get between low quality .jpeg and high quality .png.
In my experience, it's more of the same stuff. So long as you have a render that has some flow to it, all you have to do is utilize the same techniques you've always used. You can switch out the anime girl in your first signature above with the face of a cute girl, and get the same sort of result. The blending is all the same (smudge, ripple, destruction brushing, ect), the placement rules are all the same (rule of thirds and so on), and the theory is all the same (contrast, complimentary colors, lighting, depth). There isn't that great of a difference, except that cutting out hair is even more of a pain in the ass. As always, it's a great idea to just look at what other people have done, or to look up some tutorials.
Take a look at this tutorial for what I mean on blending. It's the same techniques you'd use on an anime or game render. http://chaos-tutorials.deviantart.com/art/BEAUTY-Signature-Tutorial-86005328
http://abduzeedo.com/ - Great site for general, professional design tricks and inspiration.
http://bakarenders.com/renders/index.php - Full of pre-made renders, only have to join it for free.