What causes Viel to take on the -es ending?
It's 'man' and not 'ist' because the sentence implies that you (general) don't see much, right?
Yes. Note that the English sentence is in the passive voice and the German sentence is in the active voice.
Why not Vieles man sieht nicht?
http://german.about.com/library/weekly/aa032700a.htm (see "verb second")