The writer has the fortunate position of knowing more intimately what his or her characters look like, sound like, et cetera. Also, the writer knows his or her intent better. But, if the writer does not convey these things in a credible or comprehensible (or passionate) way, then he has failed his readers.
The reader, on the other hand, not only has the discretion to read or not read a writer's work, but can also judge from their own experiences or tastes whether or not the writer did his job well enough. For a given piece there is one writer, and an uncountable number of readers. I think the readers are the better judge. For my own writing, I depend on input from readers. It is crucial to me as a writer. I know I can write, but I want to be better.
A point of order: Slush readers and editors, though readers, do not necessarily get to be good judges of writing. They often get too many good pieces for a given submission period and simply can't publish them all.
At least, that's what I keep telling myself.