Herbert Hoover was thought of as generally ineffective as a leader, as he was unable to do anything to stop the Great Depression from happening. FDR was considered effective, and elected/re-elected four times.
It was only later on, when we were able to step away and look back with a critical eye and discover that what Hoover had proposed to respond to the Depression was basically what FDR ended up doing: ad hoc makework programs. It was only later on, when we were able to step away and look back that we could see that it was World War 2 that lifted us out of the Depression, and that employment and wages weren't really helped by any of FDR's programs.
And it was only much later on that we found out that Hoover was responsible for stopping a massive famine in Russia, and potentially saved 20 million lives, something he was unable to take credit for while he was in office.
History has been kinder to Hoover, and more critical of FDR as modern scholarship is able to get a clearer picture of what they actually did during their times in office. The same will be true for Bush and Obama, and it will be true for a number of reasons that are similar to the situation with Hoover and FDR.
First, We don't have all the information yet. A lot of the goings-on when it comes to counter-terrorism is under wraps, and will likely stay that way for a while. Just like Hoover's good works were covered up at the time and will come out later, good things that Bush did might come to light at a later date, and recolor our perception of him.
Second, We are too focused on some things. Especially with our focus on the terrorism and foreign policy during Bush, and on the economy and ISIS with Obama. This heightened sense of insecurity has given us a certain bias that forgives executive excess. While we may be somewhat forgiving of the number of executive orders Bush and Obama gave out or the creation of the Department of Homeland Security, and the TSA, and body scanners, and the curtailing of civil liberties, the assassination of an American citizen without giving him his day in court and so on and so forth...while we may forgive these things now, we may look back and say that those really were excessive, and that those in charge ought to have had cooler heads.
Third, We aren't focused on other stuff they did. Bush put money into fighting AIDS in Africa. 15 billion dollars, credited by experts as being instrumental towards turning the tide in the fight against AIDS. He also requested and got 12 billion to fight malaria in Africa. Remember that one of the critiques of US Foreign policy is that they like to help white people but not black people (think the intervention in Kosovo versus no action in Rwanda). Well, here's a great example of Bush contradicting that. But it will still be a while before he shakes off the image of Kanye saying "George Bush don't care about black people."
Fourth, and I think this is the biggest one, their efforts haven't paid off yet, so we can't render judgment. I've said this a bunch of times on reddit, but I'll say it again. We all know how history books are written, and how they link events to each other. If, in 50 years, the Arab world is full of vibrant democracies, I guarantee that George Bush will be considered one of the catalysts. His detente with Gaddafi, his foreign policy of exporting democracy, and his invasion of Iraq and the subsequent installation of a democracy there will be cited as laying the groundwork for the Arab Spring. When that happens, then folks will hail Bush as visionary. If, however, in 50 years, the Arab world is still a burning and smoldering ruins of dashed dreams and strongman-led fiefs, they'll blame Bush for having sunk the Arab world into a spiral of death. In the end, we won't know until it happens.