What I find most fascinating about The Idiot is that nothing is straight up. Dostoevsky leads the reader every which way, sometimes seemingly for no purpose other than to tell some anecdote, but in time he steers the reader back to the main story lines.
Not quite sure what the central theme of this novel would be. Most reviewers say it is about "redemption," and certainly that is a major part of the stories being told here, but it doesn't strike me that Dostoevsky is looking for any clear cut resolutions, but rather exposing what he regards as gaping shortfalls in Russian aristocratic society and the various directions persons have strayed in an attempt to seek answers.
From all accounts, Dostoevsky was profoundly Orthodox in his religious beliefs, yet there is much about Nihilism and Anarchism in his novels that shows that he took great interest in these subjects, if for no other reason than to reject them. Demons was a decidedly more political and religious novel, as he does seem to be looking for a new greater Russian state with a greater sense of faith in religion. But, The Idiot is much more personal in tone, and a much more enjoyable novel in that regard.
I suppose The Idiot can be compared to Don Quixote, but it is by no means a direct interpretation. Prince Myshkin may be comical in appearance and mannerisms, but he is certainly not comical in nature, and understands all too well what is going on around him. There is so much lurking in the various subtexts as well, like that between the Prince and Aglaya which seems to go largely unsaid. He paints such vivid characters that they take on lives of their own within the novel.
I would agree with this reviewer that General Ivolgin is much closer to Quixote, but I thought the connection between Myshkin and the General to be much looser than the reviewer implies. The image above is from Nekrosius' play