A guy like Thune makes a guy like O'Malley look like a firecracker. He just doesn't have the personality necessary for someone running these days. On top of that - as was mentioned before - I don't think he wants to run.
He'd probably over-perform in Iowa and maybe Minnesota, but I don't think his Midwestern appeal is the same type of appeal that can grab a completely different series of Midwestern states such as Ohio, Michigan, etc. It'd be like saying that Rick Perry would do well in VA because it's the South.
While I agree with you on most of this, I disagree that Thune is not charismatic. He has the potential to be-he is very well spoken, presidential in his looks, and has ambition. But that ambition lies in the Senate, not the White House.