To Bek: The best way to learn letter structure is to start with basics. Draw block letters similar to the letters on your keyboard, but SEPARATE each bar. For example: The letter "A" has three bars (in this case). draw each section separately to make a good clean letter (this is similar to "starting with a circle" when drawing a cartoon character or something). Then start experimenting by bending your bars, changing the width of the bars, how your letter is going to balance, etc. It takes time, but you'll be a much better artist (not graffiti artist; artist) once you start practicing this.
Eros- I'd like for you to read the same crits as bek. You need to learn more about letters and how they're formed before you try to contruct throws. Maybe even work on handstyles for a while. That's really most of what I do. Handstyles and straight letters. I'm not a good graffiti artist by any standards (like Phil said, we all are in the toy forum), but I think I have enough respect with my handstyles to give crits. Making solid handstyle can help you later construct any aspect of graffiti (throws, straight letters, pieces, anything). Not to sound cheesy, but handstyles are the BASE of your artistic style. So, hey, it might not be terrible to work on those for a while.
Task- getting slightly better. I would still shy my eyes away if I saw that on the street. It might help to draw some throws on graph paper to make sure each letter is the same width and height (approximately, at least) And you just have ugly bars all around. No structure. I actually think your bottom throw has more potential. May just be me, though.
TAK- The K is awful. I think you need a block letter K rather than the bubble nonsense you have now. The TA aren't that great either, the bars are not consistent.
KRSD- Same advice as everyone else.
And I already got a few crits on this, but I'll post so the rest of you can share advice since I just wrote all of that shit. Sorry for the blur.