Bush and Hitler are similar in respect that they are both fascists. Both have rendered their respective legislatures useless(by force and capitulation) and are autocrats. If Bush decides to invade another country(Iran, Syria) the Congress will not stop him. There are no checks and balances left within the American system. He decides, Congress gives in.
The authorization to go to war with Iraq and the Patriot Act are worded in such a broad way that it gives him nearly unchecked power. Anything Bush chooses to do can done under the guise of National Security. The unitary executive didn't start with W, it has been a steadily evolving process since Harry Truman signed the National Security Act in 1945. But Bush has kicked the ball so far down the road to fascism during his term that there is very small likelihood of turning back.
Impeachment is the responsibility the Congress has to make sure the President upholds the laws of the Constitution, and they will not do their job as they watch what used to be a democracy crumbles.