They really seem to see things different on the issues. I don't think he is a bad pick. I just think there had to be someone else out there that Hillary had more in common with than Kaine.
Lets run down a few things here.
Tim Kaine is an outspoken advocate of free trade. Hilllary said the Trans pacific partnership was the gold standard of deals but she has come to oppose it.
Tim Kaine opposes the death penalty. Hillary supports it but in limited circumstances.
Kaine wants stronger envioremental policies to reduce pollution, Hillary wants more done to protect infrastructures from pollution.
On same sex marriage Kaine backed extending same sex marriage laws to having the same rights as a tradtional marriage. Hillary called in '96 for strengthening traditional marriage but she has gradually shifted on the issue.
The most obvious one is the abortion issue. They see differently on that.
Not saying he is a bad pick but they seem to have different views on quite a few issues.