I think what makes it a classic is the profound effect it can have on the reader. You cannot help but feel almost claustophobic reading this novel, such is the suffocating atmosphere of oppression that Orwell creates in the setting. Just as we might connect and contemplate some of the novel's ideas to the societies of Nazi Germany or Stalinist Russia, at times we might also be struck by similiarities in our own society to Oceania.
The tactics used for 'rehabilitating' citizens are particularly disturbing as they go beyond simple brutality to highly effective brainwashing. Winston's experiences at the hands of O' Brien and the secret police is the most chilling aspect of the novel - by utilizing its best resources a state can indeed 'get inside us'.
1984 is a novel which achieves much in moving readers to compare the nighmarish world of Orwell's creation to the realities of the society in which they live.
This is a good question because the qualities of 1984 as a novel are, let's say, questionable, yet it remains a great book.
I believe that this is true because of the power of the book's ideas, which are referenced very often in Western culture. 1984 describes a world that is frighteningly familiar and which presents us with a warning, if we want to take it that way. If we don't want to take it that way, the book presents us with a view of a world where politics have taken over society in ways that diminish humanity.
That being said, there are flaws in this text in regards to plot and structure. The long political statement made in the last third of the novel is out of place and disruptive to the narrative, even as it stands as the book's clearest statement on the political ideas it expounds.
I think that the manner in which Orwell has configured the role of government and its control over people is what makes 1984 a classic. Even if the work is disparaged because of its dystopic nature, Orwell's book makes it clear that there is a precarious relationship between individuals and the government. One reason why Orwell's work can be considered a classic is because it shows how government can seek to consolidate its control over its citizens. The use of technology, the drive for conformity, the punishment for dissent, and the need to constantly immerse a nation in war are all ways in which "the state" becomes healthy at the cost of its people. While Orwell's work is not meant to be taken literally, it is one that speaks to how modern government seeks to take power away from citizens and will do so in manners that might not appear to be disempowering, but end up serving that effect. In this statement about modern politics, the work has to be seen as a classic. It has changed the way in which individuals view their government and how their government views them. This is what makes the book so powerful and so much a classic.