I have said many times that I don't think that elections (and mainstream politics in general) are the way to get meaningful change. There are reasons to vote for one candidate over another, but I maintain that even the most progressive public policy proposed by the most progressive candidate in a capitalist state cannot solve the problems that are inherent in a capitalist state, but only prevent some of the worst effects of those problems.
But like I said, there are some reasons to vote for one candidate over another, and I'd like to discuss them here. First we have the standard liberal causes:
Minimum Wage. If the market could simply enslave workers, it would. Since it cannot, it will instead pay them as little as it can (while extracting as much profit as it can from their work) without them leaving. A capitalist market will never pay people fairly for their labor. It will always underpay those who are employed and overpay those who employ. Minimum wage laws cannot eliminate this imbalance, but if the minimum is set high enough they can ensure basic living conditions are met for workers.
Social Welfare. Capitalism relies on unemployment, and underemployment, to keep a competitive job market and maintain low wages so that the owners can make larger profits. The only force comparable in power to the market economy is the state, and having profit redistributed through taxation to prevent homelessness and starvation is one of he few antagonisms between those entities that we can exploit. Normally the state and the economy are in lockstep; despite their taxation complaints, the wealthy rely on the state to protect their "right" to their wealth. Any wedge that can be driven between them weakens both, and in this case also prevents real suffering for those harmed by both.
Abortion. The government does not recognize all of our rights, but we do not need them to take away those that they presently do. Whether legally justified on grounds of medical privacy or not, women have a right not to have their bodies used as incubators for fetuses without their consent. The right to abortion is not a special right, but the same right to liberty and personal security that freed the slaves. It is a right worth fighting for.
Affirmative Action. To reject affirmative action is to reject basic equality. We live in a society still deeply scarred by racism and sexism, and until those scars heal it takes systemic measures to cancel out the privilege white male people continue to enjoy in educational and occupational opportunities.
Anti-This-War. I have been critical of this position, and I do think that it is one often taken for selfish reasons. All major party candidates believe in the desireability of a strong military and periodic interventions (perhaps labeled "humanitarian") to make sure the world knows that if they don't go our way we can destroy them. Bill Clinton did it in Bosnia while ignoring East Timor. George W. Bush did it in Iraq while ignoring Darfur. But the opposition party must oppose something, and to the extent that at this particular moment the primary show of force the United States is involved in concerns Iraq, a candidate who would take soldiers out of Iraq is preferable to one who would not.
Then we have the handful of things that are unlikely to be rallying cries for a major party, but that if they were on the table, would be worth supporting:
Recall, Initiative, and Referendum. If we must live in an ostensibly representative oligarchy, the best we can hope for is to have more oversight on what the decision-makers decide. As it stands, there is very little pressure for elected officials to act in their constituency's interest, except to the extent that they can be elected. Because elections are relatively infrequent, the people have no means of putting pressure on an official over any given issue save letter-writing. But if elected officials at all levels of government could be recalled at any time by their constituents, it would go a long way toward shifting power into the hands of the people. By the same token, any measures which increase participation by the people in the actual decision-making process, such as initiative and referendum, are beneficial to strengthening the popular voice and weakening that of the ruling class.
Maximum Wage. Having a legal maximum wage tied to the lowest paid at a company acts to mitigate the injustice of capitalism in one major way. Because those in decision-making positions have no choice but to raise the wages for their employees any time they themselves wanted a raise, it prevents pay gaps from increasing any further than legal ratio. Every time the CEO gets a raise, the janitor gets one, too.
Beyond those issues, and perhaps a few others, there is no compelling reason to spend more than a moment organizing and supporting mainstream political figures. It's a simple matter of opportunity cost. Every minute (and dollar) spent supporting a status quo centrist like Barack Obama is a minute (and dollar!) not spent actually building a better world through democratic organizing and education, cooperative businesses, unions, and direct action.