Joe Biden and Sarah Palin.
Of Biden I said before that he was a, "Smart/tough/egg head senator from Delaware who can never gain traction and spent much time defending himself and/or making jokes during the primary. Nice guy, only good in the senate." I still stand by that and in many ways he is a risk. He talks in a folksy stream of consciousness that is prone to gaffs that are killer in today's politics. But, he does know everyone, and has been involved in everything since 1973. So he is a practical choice - he is someone Obama wanted to be his VP when he is president. If he had wanted a political pick, he would have chosen Hillary.
So in the end, Biden leaves Obama strengthened but open to future problems.
Now for Sarah Palin. First off, who the heck is she? She is a 44 year old 1st term governor of Alaska, who has been in office for less than 2 years. Before that, she was the mayor of Wasilla Alaska, pop 8,471. She is clearly, if not exclusively, a political choice. No one knows who she is, and she has zero experience. But, she is a woman who will attract some Hillary voters, and she is a strict pro-life conservative who will assure conservatives that it is OK to vote for McCain. The only exposure she has had at a national level is on the issue of oil and energy (Alaska's main source of income), and the fact that she is young. Here is a web ad made for her last July. She is an amateur.
Palin is an awful pick. McCain would have been wise to chose Tim Pawlenty, who is a dangerous attack dog and cool operator. Palin is inexperienced both in policy and politics and I can't imagine her holding up well under pressure.