These are just my opinions. Feel free to add or object.
1- McCain tried to stay in the middle when he should have gone right. McCain wanted to be Mr. Nice Guy and refused to go negative. Obama stayed left the whole time. By staying the middle, McCain got run down.
2- He ignored conservative outlets, mainly talk radio. Even George Bush made the effort to go on Rush Limbaugh's show prior to his reelection in 2004. McCain was too busy going on The View, Saturday Night Live, and David Letterman, while making a fool of himself each time. He dined with the devil here and didn't bring a long spoon.
3- He failed to denounce the media for their treatment of Sarah Palin. McCain could have looked stronger and more confident in his choosing her. Instead he goes on SNL three days before the election and partakes in a sketch that mocks her.
4- Conservative Republicans never embraced him. When Mccain became the apparent GOP nominee, so many on the right were bewildered and upset. The Dems had a win/win situation with Clinton or Obama.
5- His stunt of suspending his campaign to vote for the 700 million dollar bailout backfired. McCain should have opposed this right form the start. A overwhelming majority of Americans opposed it. McCain should have called out Obama on it and make clear that cooperate crooks would not be rewarded. He could have beaten the Dems at their own game.