Seems similar to what happened to Woodrow Wilson, Harry Truman and LBJ. They left office deeply unpopular after perceived foreign policy failures, but history was more generous with them. It is yet too early to say that history has spared W, though.
I think Bush's "legacy" has been helped by how incompetent and ineffective Obama turned out to be. A strong or successful Presidency would have provided an immediate contrast to the Bush Presidency. As it is, there's little to no clear difference.
The criticism of Bush's Presidency isn't ineffectiveness--he was effective in getting many of his favorite things through Congress (military action, multiple tax cuts, Medicare Part D, etc.), and nobody disputes that. The criticism is directly related to walking away from two wars without any sort of exit strategy, total incompetence in crisis management (Katrina), and leaving the economy in the crapper.
Bush's rating as a President is still in the basement, and that's why.
