What's so important about Guy Montag?
Arguably, what makes Guy Montag so important is the fact that he is ordinary. He has a respectable, though not extraordinary, job as a fireman, lives in a modest home and has personal debt (because of the additional parlour wall which Mildred has installed). He is also married but feels very disconnected from his wife as she spends so much time listening to the walls and her Seashell radio. As such, Montag is the sort of the person to whom the reader can relate because they likely share some similarities.
Because Montag is so ordinary, Bradbury's message is made all the more poignant to the reader because it implies that everybody- regardless of background or status - needs books and education in their lives. Moreover, Montag's experience shows that books can bring happiness and fulfilment to the most ordinary of people and this is why censorship must never prevail.
I think that what is important about Montag is the fact that he symbolizes the possibility of human and societal redemption. He is part of the government's effort to stamp out books and thinking and yet he becomes involved in trying to recreate a more human way of life both for himself and his society. I think that he gives us hope that people and societies can mend their ways and redeem themselves.
Primarily, Montag is the hero/protagonist of Bradbury's novel, so thats the first reason/cause of his 'importance'. Then, secondly, he represents in my opinion, a sort of 'dystopian everyman' who lives in a nightmare scenario but wishes to be out of it too; and fnally, as pohnpei397 states above, he as protagonist and everyman, emerges as a symbol of hope and redemption, and even more than that possibly, as a budding voice of dissent and revolt.