The United States of America is a Representative Democracy; as citizens it is our responsibility to elect the officials who will propose and enact our laws. I am never ceased to be amazed at my fellow citizens who willingly relinquish the right to vote. Not only that, they seem to wallow in some demented sense of self-righteousness loudly proclaiming that all politicians are the same or one vote doesn't matter. If you seriously think there is no difference between John McCain and Barack Obama, then you haven't been paying attention. Or you're too lazy or too indifferent or too ignorant to take the time and look into the facts about each candidates and their positions.
As for "my vote doesn't matter". The 2000 election which Gore lost by 527 votes should be enough to disprove that excuse. Of course Gore actually won the popular vote and might have gone on to win the electoral vote as well if the Supreme Court hadn't handed the election to W, but that's another story.
There are people who honestly don't care about the election one way or another; diffidence is worse than ignorance in a democracy. This nation was forged from the blood and sacrifice of men and women who gave their lives in pursuit of self-determination; they died for the right to chose their destiny. If they could have foreseen the lack of interest their descendants have in the political process, they would have chucked it all in and sailed back to England. There is evidence to suggest that this election is different; record numbers of new voters are waiting in line to cast ballots, and there is a real sense that this is a defining moment in the American history.
If you don't vote, you lose the right to complain when elected officials make decisions you don't agree with.
"The vote is the most powerful instrument ever devised by man for breaking down injustice and destroying the terrible walls which imprison men because they are different from other men.” - Lyndon Baines Johnson, thirty-sixth President of the United States.