When people run for President of the United States of America, they spend a lot of time making speeches, informing voters of their goals and their vision for America. They tell you what they are going to do in the future. Then the opposition usually takes it upon themselves to tell you what they have done in the past.
However, you can get a glimpse of the type of person the candidate is, and the type of decisions he (or someday she) will make, when confronted with choosing a running mate. You see what they consider important,and what type of person they want running with them.
In the case of 2008, apparently Senator Barack Obama considers it important to be connected to career politicians and seen as a Washington, D.C. insider. For a candidate to build his entire campaign on the words "hope and change" to then pick a running mate who is so clearly the antithesis to them has to be a bit... deflating for his followers. Senator Joe Biden has a long and distinguished career as an elected official - emphasis on the long, as he has been an elected official for 36 years. He's been a politician for longer than - 40%? - of the voting age population has even been alive. Apparently Barack Obama thinks he needs to reach out more to the liberal, white, upper-middle class, East Coast, straight-Democrat-ticket voters of America.
On the other hand, Senator McCain caught everyone off guard in his masterful pick of Alaska Governor Sarah Palin. Senator McCain, with his pick, told America this - I value women, I value blue collar workers (her parents were and her husband is). I value hard work, I value family (she has 5 kids!), I value Life, and I value the ability to stand up for what's right (her first act as governor was to put an end to the state of Alaska's participation in the ridiculous "Bridge to Nowhere," calling it a "waste of money.")
In other words, McCain showed America hope. And change. And he didn't resort to mere rhetoric, but demonstrated it through actions.
Like I said, the first decision is a telling one.