The Obama campaign liked Kerry well enough that he was very prominent as their surrogate - even on the last MTP. They also did not vet his exceptional 2008 convention speech. So, it does mean they trust him.
He is very very well postioned to likely be the most powerful liberal/progressive Senator (the Kennedy wing) in the future. It also keeps Kerry as an independent voice. Kerry spoke constantly and enthusiastly this fall in MA about how "he didn't get in to government just to stop bad things, but to do good things" and that with the Seanet majority they were likely to have and a President Obama would let him do things he wanted to do.
There is also the fact, that unlike many politicians Kerry doesn't owe anyone his seat because they "gave' it to him - he was not the media or party favorite when he first ran for Senate. The one group of people he does owe is the people of Massachusetts who have voted for him - for 5 terms as Senator, 1 as LT GOV, and as President. He may feel that the likely loss of one beloved Senator will be hard enough on the state.
Kerry will have the chair of the SFRC, and he seems enthusiastic with what he can do with it - speaking of hearings on things like:
- the global warming treaty that will be written by December 2009 (Kerry is going later this week to the global warming talks in Poland, heading a Congressional delegation - Obama is sending no one and has said he will be briefed by the Congressional delegation. Kerry was praised even by the Bush Bali team for his major contributions.)
- an international solution to tax havens like the Cayman Islands. Kerry in a Senate Finance committee hearing on this spoke of it as both a national security issue (what you can't see can be a problem) and as a basic fairness issue - as others have to pay more becuase of this. Any Edwards' fans who liked that he would fight coruption might be impressed by Kerry, who is the real deal on this - here's a link to a Q/A that mentions it -
http://www.youtube.com/watch?v=m5ZlrbkdBNk There is likely no one else as able to do this as Kerry is the only person on both the SFRC and the Finance committee.
- It would also seem that as the Geneva Convention is an international treaty, they might be able to have hearings - on complying going forward. Few people could or would do this - and Kerry is very likely the best.
In addition, from his other committees, he has many things he has been working on - most notably a big high speed rail bill that he has already gotten substantial bi-partisan support for.
Joe Biden had long ago ruled out taking Secretary of State under either Obama or Clinton. Kerry's situation in the Senate is more similar to Biden's than to HRC's, where she really had no chair and little seniority. Biden has more seniority, but (before Biden was VP - when he said that) Kerry had far more of a public presence. Kerry has also always been more willing to be independent from the party when he thinks it is the right thing to do.
There have been smears against Kerry's role in the Senate - and they've come from both Republicans and Democrats. The truth is better seen by the fact that his peers waived the rule limiting Senators to 2 "Super A" committees, to let him switch from the Banking Committee to the prestigous Finance committee, without giving up Commerce and Finance.
""Super A" committees. Republican Conference rules limit party members to service on only one of the so-called "Super A" committees--Appropriations, Armed Services, Finance, and Foreign Relations. Democrats observe the same practice for five committees--these four plus the Committee on Commerce, Science, and Transportation"
http://www.senate.gov/artandhistory/history/common/briefing/Committees.htm