Throughout my time as a student and editor for a student publication, I've found verbosity to be the most imperative issue for amateur writers. Whether frequently imposed word-amount requirements or nonconstructive analysis is the culprit, I'm not sure; however, it is the most detrimental issue to quality writing. The general sense that a particular piece of writing is "good" comes from--not diction, syntax, or grammar--but rather the content. "Fluff," as verbose writing is often called, when examined out of the context of the writing can appear to be very thoughtful and creative, but when placed back into a particular piece where the same idea has already been conveyed several times though in different forms, the writing loses quality from the redundancy. Length, it seems, is often mistakenly assumed a demonstration of intelligence, though a person who talks a lot doesn't always have a lot to say.
Ernest Hemingway wrote a short story entitled Baby Shoes, which he considered to be his greatest work. Consider the content-to-length ratio in his story:
"For sale: baby shoes, never worn."
Succinct. Powerful. Emulate that.