I've been a lurker for a bit and finally decided to register to the site and I am glad to comment/critique.
I am thoroughly impressed with all the pieces from both beginners as well as those on a professional level.
But in the 2 days I have been on here the #1 question people ask is what can they improve in their pieces. Now this is NOT the solution to every piece, but for beginners (like me), this will help a ton.
Please do not forget this because I have seen alot of pieces where the perspective involves out-of-nowhere vanishing points that just do not add up with horizon.
I am 100% sure all of you guys understand this, but please remember Perspective is one of the main elements of drawing that must be correct.
The Horizon line is eye-level. Remember this: Again you probably already know but I want you to say this every-time you critique your own piece before submitting, because if the perspective is not accurate, it throws off the rest of the drawing elements.
Andrew Loomis Recommends this Book in Learning/Improving your understanding. Just read the first 15 pages of Google Book's Preview and I am 100% sure that those pages alone will help any beginner understand perspective much easier.