1. Big margin = big mandate. The popular vote doesn't put anyone in the White House, but it affects what presidents can do when they get there. Want Obama to be able to actually do the stuff he's been talking about? Pass universal health care? End the war? Then we need a landslide.
2. The other things on the ballot matter! For example: Congress. Without more support in the House and Senate, Obama will have a hard time getting progressive laws passed. Plus, there are other important local races and ballot questions in some places.
3. If you don't vote, everyone can find out. Voting records are public. (Not who you voted for, just whether you voted.) Pretty soon, finding out whether you voted could be as easy as Googling you.
4. Help make history. You could cast one of the votes that elect the first African-American president. If we win, we'll tell our grandchildren about this election, and they'll tell their grandchildren. Do you really want to have to explain to your great-great-grandchildren that you were just too busy to vote in the most important election in your lifetime?
5. People died so you'd have the right to vote. Self-government—voting to choose our own leaders—is the original American dream. We are heir to a centuries-long struggle for freedom: the American revolution, and the battles to extend the franchise to those without property, to women, to people of color, and to young people. This year, many will still be denied their right to vote. For those of us who have that right, it's precious. If we waste it, we dishonor those who fought for it and those who fight still.
Live your values. Love your country. Vote.