If you compare it to your reference I think the biggest difference is the contrast. The original has a much higher contrast. I would recomend posting your art on the pixel joint forum. There are alot of helpful people there. 

and second advice - please don't scale image, there is zoom on PJ :)

Thank you for your advice, next time I'll try to use less colors and use dithering effect on them.

137 colours - this is too much IMHO.

Here's my tips:

I think you can improve the work by mixing colours (dithering). Also, using a palette which has few, saturated colours can produce a much vivid character.

Otherwise, everything else such as the concept and outline of that goblin is nice and neat (althrough there are some jaggies and 2-pixel thick outlinines that can be cleaned).

Pretty impressive for a first pixel art.