CQPolitics.com:
Biggest surprise: Minnesota Republican Michele Bachmann insisted her reputation as an outspoken social conservative put her right in line with voters in Minnesota’s 6th District, which reaches from suburbs of Minneapolis-St. Paul to the city of St. Cloud.
I thought the anti-Republican climate would seriously hinder Bachmann, as would the apparent strength of her opponent, Patty Wetterling. The Democrat ran a viable campaign in 2004 against the Republican incumbent, Mark Kennedy, who left the seat open this year for a Senate bid that failed.
Wetterling, well known for years as a child safety advocate who had suffered the personal tragedy of losing a son, also had gained visibility late in the campaign as a sharp critic of the Republican leadership’s handling of the Foley page scandal.
Well, I wouldn't have ranked that as "biggest surprise," however Bachmann's sizable margin of victory over Wetterling
was a surprise. So is the fact that Bachmann ran better* against Wetterling than Kennedy did, considering Kennedy's reputation as God's gift to the Republican Party. Who predicted that?
*I don't know if she ran better in absolute terms, but considering she was a non-incumbent running in a much worse year, she did relatively much better than he did.