First off, I'd like to say nice job! I was very interested when this was in my inbox, partially because it's different from what you normally draw. But anyway, on with the critique!
Lineart: The lineart is nice, very smooth, but a little rough in some places. Overall, very nicely done.
Characters/Anatomy: Jack looks really good, and most of his anatomy is flawless. The only thing I really noticed is the position of his head on his neck--they join at the very bottom of the skull. The joint should be toward the back of the skull (you could search deviantART's stock image section for pictures of the human skeleton for reference of this). His legs from the knees down are also a little short.
Background: minimal background work. I'd suggest some sort of shading on the moon, or some smaller stars mixed with the larger stars. You could even put a slight gradient on the sky.
In comparison of your other works: I can tell you've put some good work into this, and it's paid off! The lineart especially impresses me
Possible means of improvement: Mostly what I mentioned before of background work. Also, do a little research on shading, particularly backlight (again, the resource and stock image section of dA might become your best friend ). The only other thing I can think of would be for you to really practice expressions.
Overall impact: Definitely showcases your versatility as an artist! A very lovely piece of fanart, I must say ^^