Sarge - Keep the focus on the important subjects. Accomplish this by using less detail and weaker colours on background stuff. The 3 tubes coming out of the TV thing are ruining the drawing for example. The background would be much better without such a strong red. Small mistakes in proportions (hands for example).
Scout - You need to learn basic perspective. Also the demon on the ground looks like it's made out of cloth: use color to give shapes to objects rather than an uniform colour and pencil tracing to set a difference. Zombie left arm. Far away zombie should be smaller unless it's a 8ft zombie.
Medic - Again terrible use of perspective, noticeable whenever you have to draw anything with parallel lines. You made a mistake with the armour / legs and it just makes the whole drawing look strange. I liked what you did on the floor / background on this one. Still you didn't need so much detail on the top wall.
BFG - Argh... So much color. Torso seems too broad again. Weapon looks like it's made of plastic except the small donut.
Marksman - Skeletons are really hard to draw, I know. It is okish to be honest. Severe flaws in perspective again. Again ARGH... color, eyes bleeding. Main character looks ok (except it's furry of course :P).
Badass - Well, he could have looked badass if not for the drastically disproportionate arms (and the fur of course). Color treatment has to be uniform throughout the drawing, otherwise leaves you with some places looking really well (eyes, gunfire, torso sides) but crap overall.
Chaingun dude - This one looks ok if we forget the choice of colours and the fact that the chaingun looks like a plastic toy (and the fur). Dead imps look really cool. Errors in perspective but not too bad.
SNEAK Attack - I really like the 'furry' on this one, well proportionate and looking to have a goal. The Vile has way too little detail considering he's on the center up-close. I expected to see little details like tendons, Imperfections, ketchup stains at this distance.
Raptor - Same as before, although the demon looks better than the Vile. You have a problem making weapons look badass but that's probably because you have trouble making stuff look like metal rather than plastic (using yellow doesn't help either).