There is some good stuff in here! You definitely need to do more studies, but right now your creativity is your strongest weapon. Your anatomy isn't terrible and the poses are stronger than what I usually come up with. There are two areas where I think you could improve fairly easily: Line, and composition.
The weight (thickness) and value (darkness) of your lines are the same all over. This makes your sketches look flat and less interesting. Try varying your weight and value as you work to see what effects they have. One technique you'll see a lot is to use darker/thicker lines to silhouette your subject, and use thiner/lighter lines inside that border. Also thiner lines can be used to show what is farther from the viewer. Also don't limit yourself to simple lines. Lines can be hard or soft, a line can be the boundry between areas of different color or value, you can even describe things by the absence of line.
Okay, that is as close as I can come to constructive criticism. Just keep working on the basics, expanding your knowledge, and most importantly don't give up your creativity. Creativity is the fire, all the rest of this is just fuel.
Composition is a harder problem. Basically it is the use of contrast to control where the viewers eye travels. Prometheus's magnum tutorial has a lot of useful ideas about how to acheve this: http://www.itchstudios.com/psg/art_tut.htm
Your more finished drawings seem to have a lot of small details all competing for focus. Also there isn't any difference in value or line between the forground and background. Even sketches can benefit from composition, to see a great example of this in action checkout Lungbugs SB:http://www.conceptart.org/forums/sho...ht=lung+sketch