In Walden, what did Thoreau believe is important to life?

Thoreau is a Transcendentalist author.  The
best way to describe them is to more or less say that they are extreme Romanticism authors.
 Romanticism is a literary time period and genre that has some key features to it.  Carpe diem,
emotion over reason, and a deep reverence to nature are all key features of romantic literature.
 Transcendentalists take it "one step further."  Simply put, a transcendentalist
believes that a person can gain special, transcendent knowledge about creation, god, faith, etc.
from being close to nature.  

Thoreau's  is his written
explanation of how he spent his time living alone on the banks of Walden Pond in Condord, MA.
 I've been there.  It's beautiful.  There's even a mock up of the house that he built for
himself to live in.  It's a simple, small affair.  And that is the key to Thoreau
and Walden.  Simplicity is paramount to achieving true happiness, peace,
life, and transcendent knowledge.  Without simplicity, it isn't possible to live life to the
fullest or really be able to be an integral part of nature and man's surroundings.  Thoreau best
sums it up with the following lines.  

I went to the woods
because I wished to live deliberately, to front only the essential facts of life, and see if I
could not learn what it had to teach, and not, when I came to die, discover that I had not
lived. I did not wish to live what was not life, living is so dear; nor did I wish to practise
resignation, unless it was quite necessary. I wanted to live deep and suck out all the marrow of
life, to live so sturdily and Spartan-like as to put to rout all that was not life, to cut a
broad swath and shave close, to drive life into a corner, and reduce it to its lowest terms, . .
.