I've already stated why I didn't support the war. And in the comments, I have shown why it was Rumsfeld's rationale that I cared about, not Bush's (Rumsfeld's rationale, not Bush's, is the one that determines how the war was fought), and why Rumsfeld's rationale casts a bad light on Bush as a leader.
Now onto the mistakes made during the war.
Rumsfeld insisted on invading Iraq with a very small number of troops. His generals told him that they needed more troops. The generally accepted Powell Doctrine demanded more troops. Whenever the strategists ran best case and worst case scenarios, Rumsfeld always chose to send less troops than was necessary to win in the best case scenarios. But Rumsfeld had a point to prove. He wanted to prove that small, nimble military units were the future of the American military. And you know, he may be right. But that future is not yet now. He sent in too few troops. He did it against all advice. He did it to prove a point. As a result, Iraq is (as far as we can tell) far worse off than it would have been had he sent more troops initially.
As a result, borders were not secured, infrastructure was not secured, libraries and museums were nut secured, weapons depots were not secured. Polling places will be difficult if not impossible to secure.
Insurrections were difficult if not impossible to put down.
We did not have enough staff or training at Abu Ghraib.
Nobody thought that we wouldn't be able to topple Saddam. But the far more important goals were to stop the human rights abuses, to win the hearts and minds of the Iraqi people, and to spread democracy. Rumsfeld didn't run this campaign with any of these more important goals in mind. As a result, we will not accomplish these goals to nearly the extent that we would have if we had planned this war appropriately.
This is largely a critique of Rumsfeld. Why should Rumsfeld's failures prevent me from voting for Bush? Two reasons:
1) Rumsfeld will continue to run this war as long as Bush is president.
2a) Bush should have made his vision and goals crystal clear to the man who was running this war. To the extent that he failed to do so, Bush failed to show good leadership. Alternately,
2b) If Bush made his vision and goals clear, and Rumsfeld ignored them or failed to carry them out, then Rumsfeld should have been relieved of his post. Bush's failure to relieve him of his post shows a failure of leadership for he is failing to hold his subordinates accountable for their actions.