Well, for one, you've made the eyes set at different depth.
There are subtle plane angle errors all over. For instance, the far corner of the mouth should recede with the cheek, but the texture of the beard there is exactly the same as on the near side, leading to confusion.
The color is very, very even, not enough difference to suggest a certain lighting. Focus on tone, not on color. The lips, for instance, can't be so evenly red, there should be shadows and highlights somewhere. Same goes for everything else.
If you did this from a photo, try picking photos that have more distinct lighting for your exercises. Better yet, use live models. Draw your friends, random people... Go to fineart.sk and check books by Loomis there if you haven't yet.