Here's what I noticed comparing your picture to a screencap of Medic's head from the side. (The coloring is weird because I did this in Maya and I couldn't figure out TF2's distinctive lighting style. I suggest you use Source SDK as a reference, since that's free and you can rotate and zoom in however you like.)
1) General shape - The Medic, as befitting his reputation as a mad-scientist, is a literal egg-head: the top of his noggin tapers in a very distinctive manner.
2) Facial proportions - For real people, the face is approximately composed of thirds: the forehead, the nose, and then all the rest. Medic's hairline, then, is too low, especially since the rest of the pose suggests that he's tilting his head forward.
Also, TF2's style exaggerates the size of the chin and gives all of the characters (except Pyro) Manry Manjaws of Manriness as well as slight underbites. It's up to you whether you want to draw it that way, but I still suggest you make his jawline a bit more square.
3) Lines of symmetry - even considering the shape of the Medic's head, it looks like his facial features "pull" slightly to our right (his left). It's fine to be not symmetrical, of course, but since you took the time to draw in the guidelines I thought I would point that out.
4) Faces face forward - and as the sample picture shows, the actual portion of the face that does so is surprisingly small. Medic's face looks too wide, especially his eyebrows and cheekbones. His lips also seem a bit wide (especially combined together with the above issue); in a neutral expression, the lips are not wider than the center of the pupils.
5) Less scribbling, more deliberate shading - Now, this gets more into the opinion side of things, but I feel like your hatching is very messy and it looks like it was thrown on haphazardly. If you plan to keep the shading in this style, I suggest you have these lines follow the contours of the Medic's face and eac
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