It is often said that the first major decision that a President makes is the choice of his Vice-President. If that is so, then there is no doubt that Obama has clearly made the best choice.
Joe Biden is a highly experienced, knowledgeable, centrist, and able Senator, who would be able to step into the Presidency if it were necessary literally in a heartbeat. He is ready to go, and almost no one disputes that.
Sarah Palin, on the other hand, is a virtually unknown Alaskan politician, who has been a controversial mayor of a very small town, then governor of Alaska for some 18 months when she was chosen by McCain. After a rousing convention speech, it has all been downhill from there. Not only is she inexperienced, but her knowledge of national and international issues is so limited that the campaign has kept her from talking to the press. And on a issue not much talked about actually, she is as far to the right politically as you can possibly get in the Republican party. It would be the equivalent of Obama having chosen Democratic Congressman Bernie Sanders of Vermont, a socialist.
Palin has caused many conservatives to reconsider their vote for McCain, because who wants an unprepared, woefully inadequate politician in the most important and powerful office in the entire world. Many people consider Palin's choice to be a reckless one, disqualifying McCain for the Presidency.