I tend to think that there are two strongly defining characteristics of Winston. On one hand, I think his normalcy and his "anyman" quality is undeniable. Orwell's giving him the most common last name of "Smith" helps to bring this out. Yet, it lies in Winston's most basic tenets. Flashbacks of his past, the desire to seek rebellion on the smallest and largest levels, as well as the consciousness that seeks more than what is present helps to bring out the fact that Winston is really no different than anything else. It seems to be a statement that he is the hero of the novel, the person in which the hopes for change rest. There is nothing extraordinary about him and this is what makes him perfect for us, the reader, to place our desires for change. Along these lines, I think that his authenticity is probably another dominant trait. Winston is genuine about his disdain for Big Brother, his desire to be a voice of dissent, and his feelings. This is probably why he fails to recognize O'Brien's duplicity.
Winston's dominant characteristic is his ability to assimilate to his environment. In the beginning of the novel, Winston has become dissatisfied with the Party. Even though he feels that he is walking a razor's edge, he can live the double life of Party comrad and rebel/Juila's secret lover simultaniously. When he commits thoughtcrime in the beginning he is aware of the dangers, yet fluidly covers his guilt by quietly helping his Party loyal neighbor with her plumbing. As his relationship with Julia flourishes, part of his attraction to her is her ability to live a double standard without care. Of course, when this stack of cards collapse around him and his personal rebellion is put down, he once again assimilates to the blind loyalty he so desperately tried to eradicate from his psyche in books 1 and 2. As anticlimactic as it seems, Winston moves from disgruntled party member, to rebel, to a lover of Big Brother seamlessly