Attempt at Digital Painting

I know you've had tons of advice on this one already, but I'm here to toss in my few cents.
Anatomy - there's a lot wrong with it. For the most part, it's just too contorted. Try making a similar expression in the mirror and see the way your face bends. Good structure on the nose, but the ear is particularly undefined. Ears are hard to grasp (too many ridges!) but when you learn where they all go it's actually fun to draw them in perspective! The neck is lacking an esophagus (OMG HE CAN'T SWALLOW/BREATHE) and the clavicular and sternal tendons (ZOMG HE CAN'T MOVE HIS NECK!). When you add these, it adds massive realism. I used to struggle with necks, but now I can draw them from imagination. You can check out my newest drawing - "Mother Confessor" - to see what mine now look like. Keep in mind that my necks used to look like cylinders, as evidenced by my older deviations. Oh, yeah - he also should have some folds between his eyebrows from making that skeptical/playful look. Try doing that with your own eyebrows in the mirror and you'll see what I mean.
Background - I'm pretty sure I recognize that stock from when I used a similar one in a title I animated in Adobe AE. Try to make the background emphasize, or work with your subject. You can try making the edges of the canvas black, fading to colour near the centre.
Shading - you seem pretty unsure about it. This improves with practice. I recommend doing a picture (a two-to-three-hour speed-study is fine) from reference, followed hastily by one from imagination in a similar pose, lighting, etc. This helps you build both accuracy and fantastic extrapolation. We'll have you drawing/painting photorealism without reference in no time! Try some lighter splotches on the right side of the face, the brow, right side of the chin, the lower lip (since it sort of bulges!), and the neck.
Detailing/Texturing - is an awesome try, but requires some deeper shading and colouring to actually work properly. Try to make the nicest possible painting you can using only hard-edged brushes on low opacity (will look kind of plasticy), and add texturing on a separate layer. This includes bumps, freckles, pores, moles, scars/little trenches, and wrinkles. Linda Bergkvist - who goes by *enayla on dA - has a great tutorial on skintones and shading nicely. In fact, I have saved all her tutorials on my hard drive because they are so helpful! You can find them on her site (I think it's furiae.com, but check her profile for a link). You can see the depth this technique builds in my newest digital portrait, titled "And True Love Waits."
Eyes - I'm sure you've got enough comments on this, but they are too big and too cartoon-ized. He should have lashes, more defined lids and wrinkles, black pupils, and less saturated eyes. Blue eyes only look so blue in real life because they are set against a black and white background (pupil and eyeball)! Check some stock photos of eyes to get a better hold on how the iris should look. by the way, good job on shading the eyeball, many people don't realize that it's an inset sphere, so it should be fairly grey.

Overall very good first portrait (better than mine, for sure!). I'm sure that with practice and perseverance, you can become a phenomenal realism portrait artist! I'm sorry if I came across harsh or mean - I really don't mean to!