This looks pretty sick so far! I cannot suggest much in terms of programming (because I am garbage at it) but I can say some things when it comes to the graphics. For the most part, the style of it does feel very like some older animations which is cool. I will try to group what I think.
Backgrounds/stages: I think the design of them are cool but I feel that you should make the lines a bit skinnier (which can be easy to do if you are making the graphics in flash) and try to incorporate some shadows to the objects in the background.
Design: It overall looks pretty nice! I would mostly suggest a new hud design.
Animations:
-Characters: I think the standing animation is alright, but I would think that you should have more movement in the walking animation other than the legs just moving. I would also suggest for there to be more frames in the walk cycle to smoothen it out. I would also suggest some frames in between doing movements because right now its kind of snappy as it changes from pose to pose, for example when you switch from standing to crouching, the change is instant and I think that an inbetween frame would help. The death animations are animated pretty good, but i would suggest more variety of deaths when using perhaps different weapons. Same thing I said with the walking applies with the reload animation, I think you should have the whole body be in motion when reloading other than a simple frame with the arm rotating. Generally, my overall complaint is the animations dont really feel like the style you are going for, and if you need anyone to animate something you can probably get some people on here or Hyuns Dojo to do so. (I am also open to)
-Effects: Now I don't want to be rude, but this is my main issue. From what it looks like, the blood effects are mostly a simulation(?) and it just looks kind of too thick and large (no innuendo implied.) It would look better if it the particles were small. Also by the way, I would suggest you to make the blood like circles instead of like a droplet of a real liquid. Typically I dislike the type of blood I am describing, but a lot of older stick figure animations have blood which are just a bunch of dots flying everywhere. But overall, I think it would be better if the blood was not a simulation and it to be a separate animation or sprite (By the way I could animate this for you, I am pretty good at animating effects). For the explosion (in the trailer at 0:33) feel was no offense, really sloppy and generally bad. You would be better off having an explosion graphic and have it fade/blur until it is gone. Also when the stickfigure gibs all over the place from the explosion, I would suggest you making the blood from the limbs not last as long or not make the steam of blood from them not be as forceful. Last I think it would look neat if the blood had visible droplets on the ground, kind of like the bullet shells. Also for the mussle flash, you should make it a bright yellow/white glow that lasts for about 2 frames or so and then have a puff of dust.
Overall, I think this has a lot of potential! Though I am not quite sure if this will end up on the greenlight, I would be willing to pay about 1/2 dollars or so for it in its current state.
( I am still open if you want any animations done...Please I'm bored. )
EDIT: I just downloaded the demo, I will add anything I notice while playing.
EDIT2: all seems pretty good, I absolutely love the crouch animation, it's hilarious please keep it as it is. It reminds me of
http://i.imgur.com/j6T5hLk.gif . But in all seriousness, my only real issue with it so far is the death noise when you use the knife on someone is kind of disturbing imo.
