To be clear, not all of these are bad candidates/incumbent senators. But politically they're all very weak -- probably unnecessarily so given the states they come from.
The ones that stand out are Beau Biden in Delaware, yet to be determined in Illinois, Kirsten Gillibrand in New York, and Michael Bennet in Colorado.
As you can see, each of races is fall out from the successful 2008 election. Biden resigned his seat and got a placeholder appointment to cue it up for his son Beau Biden in 2010. Obama gave up his seat in Illinois triggering the Blago/Burris train wreck, Clinton resigned her seat to go to the State Department and Bennet got appointed after Sen. Salazar went to Interior.
Defending a seat without an incumbent is always challenging to some degree. But each of these, in different ways, looks very much like unforced errors/self-inflicted wounds for the Dems 2010 election prospects.
The "best" of these appointments may well be Beau Biden. There's the obvious nepotism angle. But Biden's an established politician -- he's the state's Attorney General. And Delaware's a small state whose politics has been dominated by three of four guys for decades. It's not clear to me that there was another Dem available who would have come in to the contest better positioned. And his probable opponent, Mike Castle, is one of those three or four guys who's been running the state for forever.
Next you have Kirsten Gillibrand in New York. Maybe she'll turn out to be a great senator. But this one surprised everyone when it was announced. And she's far from the best positioned person to hold this seat in 2010. It wasn't clear when she was appointed that it would be such a tough climate for Dems. But that's life. If Giuliani gets in to this race, this one could end up looking like a senate Gov. Paterson all but gave away to the Republicans.
Next, Illinois. What can you say? It's not that Illinois would ever be a walk. But certainly the Dems position is greatly weakened by the whole Blago/Burris fiasco. And having a strong candidate in office for almost two years would have put that person in a much better position than whatever they'll face next November.
Finally, there's Colorado. I don't have a deep grasp of Colorado politics. And Sen. Bennet (D) may be a great senator. But it seems clear he has precious little political base to work with in running for office in his own right. I don't know what other Democrats were available. But his appointment seems to have put the Dems in a needless weak position trying to hold Ken Salazar's seat.
Some of these picks stemmed from personal idiosyncrasies, others unique personal situations. But all were made in the post-2008 political climate when the Democratic ascendancy seemed to flow into an endless future. And the Dems could pay a real price.