I think this is the right place.
I say that the first one, crankshaft, Is the best and could be considered final product ready.
For the other images, I would say, desaturate the colors a little bit, and especially desaturate and brighten up the second image, which looks like an orc girl. The third image could be aided with adding a bit more contrast by adding shadows like with the first one, doesn't need to be much.
Not sure about the 4th one, nice color pallet, and I am sure a bit of shadow work could improve it, but I would say make the background monster...
Ok, so I have been playing with the study of characters on backgrounds, and it seems to help if not everything is equal or the same. IE, anime character on a painted background. This works if the background is a bit desaturated, has a little less contrast, and is ever so faintly dimmer.
I point this out because it seems like with the 4th image, the monster, the girl (feminine face), and the dude are all fighting for the viewer's attention, they are all equal. I would say, the order should be from closest to farthest. So have the monster dimed a bit, while have the dude a bit desaturated. The girl, I am not too sure. One could say that supper saturated color is a bad thing, but it is also a style so... I guess a solution would be to try desaturating the cloths so that the clothes doen't steal the attention away from the character herself, the hair is fine though.
Definately recommend the concept of "final lining" to clean up the lines using one line, so that they are well defined and not fuzy. The fourth image really neads it with the three characters, but in your first image the crankshaft, it looks like you did this so try and follow that example for any piece you want to finalize. Also similar in the last image the comic, looks to have alright final lines. Having smaller detail lines that look more like sketch work looks fine so long as they are separate from the final lines and do not obscure one another, you did this well in the first and last examples that look like comics.
I do recommend to try playing with shadows and highlights. shadows can be saturated raw colors (basically the natural color of things, while highlights (such as those with the second image with the orc girl), should be very desaturated. and normally light colors should be a little bit desaturated to allow things to naturally flow from dark to light. Shadows should be of 2 types, etheir just enough to add a bit of detail to make a point of when things are in the shadow (you did this well with the crankshaft comic, but I would play with adding just a bit more ,but it is easy to over do it) or I find another option is to overdo the shadows, leaving only room for basic highlighting if any (but this works more for representing a character in a dark area or for vector artwork).
hope that was helpful.