a) Scott Brown ran a great campaign. His messaging was well done. He ran successful on the tradition security and lower taxes ideals. He won the debates.
He portrayed the "everyman" image. He worked very hard. He made it clear he cared about voters. And... he pulled the neat trick of speaking to independent moderates at the same time he was reaching out to tea party conservatives (i.e. two contradictory groups).
b) Martha Coakley ran a horrible campaign. She ran as the front runner. She couldn't of done more to show she thought she would win simply because she was the Democratic candidate. She was arrogant, detached and aloof.
The most telling story to me on the race was how she reacted when she was told Scott Brown was shaking hands in the cold outside Fenway Park. Coakley's response was "Why should I do that?". She was meeting with political VIPs at the time. This story was illustrative of how she saw the voters... she acted as if she didn't need to work for their votes.
Then there were the stupid gaffes-- calling Curt Schilling (the Red Sox pitcher famed for heroing pitching through pain in the "bloody sock" game) a Yankees fan; her strange comments about how the Taliban wasn't in Afganistan, and a poorly thought out (and obviously rushed) attack ad which misspelled the word "Massachusetts".
c) I don't think the MA health care plan has anything to do with anything. In fact, I haven't heard any one of us who doesn't think this is silly.
d) The "people's chair" line was brilliant and reinforced the arrogance of the Coakley campaign and the Massachusetts Democrats thinking that they would waltz through this thing.
e) There are people in Massachusetts who support Obama who voted for Brown because he ran, and is seen by many, as an "independent". The arrogance of Coakley turned a lot of middle voters to Brown. How this "independent Republican" thing will work out (i.e. how Brown will act once in office) will be interesting to see.