Biden is an experienced legislator with a long track record. He plays well with others, and is willing to reach across the aisle to those who disagree with him (some see this as a weakness, but I say it is a requirement). He not only believes in what he says, he is willing to listen and sometimes change his mind. I don't agree with all his positions, but I do trust opposing views will at least get a fair hearing.
He has also been in the White House before, knows how it operates, and where Obama hid the keys for the liquor cabinet. He was an engaged, active Vice President who worked with his administration to get things done. So he's done the tutorial.In the period of the campaign season itself, he has navigated the pitfalls of the campaign amazingly well. His former Democratic opponents endorse him, and endorse him heartily. He has pulled together the various wings and factions of the Democrats into a generally consistent whole. The Democratic showcase of the convention was a showcase of unity. At the best of times, the Democratic Big Tent resembles a rugby scrum more than a processional, but has he has provided leadership to pull it all together.