Much discussion has revolved around the question of just what neoconservatives are. Are they right-wingers? Leftists claiming to be right-wingers? Trotskyites? Fascists? Claims have been made for all of these. I think they all miss the mark.
Neoconservatives are centrists.
This is not an idea often put forward, but consider- they have no problem with the welfare state or regulatory state if they're the ones running it, they share the corporate liberal's horror of laissez-faire capitalism, their views on social views are comfortably mainstream, and they largely accept the standard center-left narrative about history (laissez-faire was unworkable, Progressives and the New Deal saved capitalism from itself, Truman and JFK were great presidents, etc). I've lost count of the number of times I've read Kristol, Podhoretz, and the like chastise other conservatives (and libertarians, when they notice us) for wanting something so outrageous as to roll back the New Deal or make any significant moves toward greater economic freedom or reduced government power. However rancorous their rhetoric may be, their arguments with corporate liberals on the center-left are merely technocratic quibbling over administrative details, not a clash of ideals or principles.
Remember that they parted ways with the left and the Democratic Party over the issues of Vietnam and Israel, while accepting the same basic premises on domestic policy. Keep in mind, also, that bloody international adventure to enforce "American" ideals abroad is not an exclusive province of the right, as the victims of Woodrow Wilson, Lyndon Johnson, or Harry Truman could tell you.
Now, there are many factions in politics that have good ideological reason to dislike neoconservatives- libertarians, radical leftists both statist and antistatist, paleoconservatives. But why do their counterparts on the center-left rage against them so? It's not principled opposition to violence- these are largely the same people who think interventionism is just dandy when Democrats do it. My observation is that small differences between people can actually cause more anger than big differences; to see someone come so close to the truth and yet reject it can be maddening. (Look at Protestants and Catholics in the Middle Ages, anarchocapitalists and minarchists, Trotskyites and Stalinists, etc.) I think that this, combined with the childish hysteria many liberals seem prone to, accounts for a lot of the rancor directed towards the neoconservatives by their mirror images on the "respectable" left. The heretic is worse than the infidel.