Hey Marek, The biggest thing I see is the lack of atmospheric perspective. The mountains in the background are too sharp and have too much contrast. This makes them come forward and ruins the effect of distance. Also, you need to use core shadows and reflected light to create form. I did a quick paint over to show you what I mean.
Hey Marek, This is good reference to practice from and you did a pretty good job simplifying the shapes. What you have to pay attention to is not just the values, but the value relationships. In other words, see if the value you are putting down is lighter or darker than other areas with a similar value. Example: the value on his right cheek is much darken that any of the lights on the left side of his face. You made it too light because your value relationship is wrong. This is very common with beginners, so don't feel bad :) I did a quick sketch to show you what I mean, and to give you a process to go through when you do these exercises, I hope it helps :)