I think you have potential. You have dedication which is a trait I admire of any artist, and you have the fire to learn. I'm not usually one to pick out things wrong in a drawing anymore, because I'd rather help the artist develop the skills necessary to evaluate their own work and to see the errors in it.
http://conceptart.org/forums/forum.php this forum should be of help to you, the discussion is very insightful on learning to draw.
errors/tips:
lack of balance in faces.
#15- eyes should be higher up, at about the middle line of the face. If you want to go big anime-style eyes, there should be 3 eyes about 3 eyes distance across the face. In real life, it's about 5 eyes distance.
in case you don't already, think in form, not line. draw the contour(outline of form) only after you have drawn the basic form of things.
draw with reference, but don't copy your reference. when you don't know what something looks like, look it up. one mistake I see beginners make is that they think drawing with reference is "bad."
there are no rules to making art, only tools. You should do what it takes to make great art.
But be careful with how you approach this, some things you do will be bad for your development; for example, color picking when doing digital paintings studies from reference. Ctrl+z will allow you to make mistakes with no consequence and can have negative impacts on an artist who over-use it.
http://conceptart.org/forums/showthread.php?123346-Using-photo-reference-correctly BUT DONT, DON'T BE A SLAVE TO REFERENCE
draw from life. seriously. it's challenging and will improve your observational drawing skills. build up your drawings from life with form, and don't forget to use proportion. and again, conceptart.org is going to be very helpful with showing you how to learn to draw from life.
http://www.ctrlpaint.com/form-not-shape/ focus on form, and that will improve your ability to draw from imagination.
find a goal for yourself. I don't really know what direction you want to push your work. do you want to keep doing manga-style, or do you want to go more realistic? Would you like to do both? The reason I ask this is because there is different advice for either boats. Though, it's my educated opinion that practicing realism improves your stylized work. Because when you study realism, you understand why things are stylized the way they are. Don't avoid realism because you think it's "hard." First of all, yes, it's going to be hard. But that difficulty is what makes that process so worthwhile.