I think that complaining about presidents is a worldwide practiced sport, every country has people who voted someone and after that they end complaining.
I don't understand, they elected the president, so, use political control, participate, but just complaining does nothing.
A large portion of people don't vote for the person who ends up getting elected.
Voting for somebody doesn't necessarily mean you agree with every single thing they stand for or attempt to carry out. Nobody is clairvoyant -- events occur that nobody expected and events turn out much worse than people expected...
Example: I doubt many voters in 2000 voted with consideration for the possibility of a large act of terrorism on US soil. I doubt then, by default, that many of those voters imagined a large-scale invasion into the Middle East within a few years of the election. These events were largely unforeseeable for voters; they didn't vote with these things in mind, at least not primarily. Yet, voters are still going to make judgments and criticism after these events occur because they may disagree with how these events were responded to.
The reason people complain, instead of just exercising their political power (voting) and keeping quiet as you say, is because many governments with elected representatives/leaders have a specified time in office. If someone is elected in 2008 and they have a two or four-year term, there's not a lot for people to do except judge, complain, criticize, speak up/voice views, etc. They can't vote for another two/four years, so they exercise other political powers (free speech).
Even people who voted for someone can validly complain if that person doesn't live up to their campaign promises. (and do they ever?)