Jay Cost has a characteristically astute observation about the Palin pick:
I think many people are surprised to discover that McCain intends to carry a positive message into the fall. Many of us had assumed that this election would be a referendum on Barack Obama, with McCain serving as an inoffensive backup for those too unsure of the junior senator from Illinois. Just a few weeks ago, I used this logic to argue that McCain should select Mitt Romney, as he was the best among the viable picks to go after Obama.
John McCain clearly does not share this view of the race. By picking Palin, he is signaling that he intends to win this election not just by attacking Obama, but by offering an affirmative message of his own.
What is that message? It is that he represents change, too. It's not the "drastic" change that Obama represents, but rather "common sense reform" (scare quotes reflect what we will hear from McCain-Palin, not non-partisan reality). McCain is indicating that he, too, is a candidate whose election would alter the status quo - not as much as Obama's election would, but alter it nonetheless.
Indeed, it is interesting to consider the two tickets. The fresh but inexperienced candidate is at the top of the Democratic ticket; the experienced pol who, even after all these years, "calls it like he sees it" is at the bottom. With the GOP, it's reversed. These tickets are mirror images of one another. The message to voters from McCain? If you're unhappy with the status quo in Washington, but are worried that Obama-Biden would be too drastic a change, vote McCain-Palin.