More on this later on, but if as is widely anticipated, Kristin Gillibrand is named today as the junior senator from New York, this is not a terrific outcome for progressive Democrats. Gillibrand, statistically speaking, has been one of the more conservative Democrats in the House. Moreover, she is a somewhat proud conservative, being a member of the Blue Dog caucus. In a state like New York, which is capable of electing and re-electing a very liberal senator, that's a somewhat underachieving result for the Democrats.
And I know the objection/counterargument: Gillibrand was representing a relatively conservative district in upstate New York; perhaps she will change her stripes and become more liberal upon representing the entire state. I don't doubt that's true to an extent. But Gillibrand's R+3 district wasn't that conservative by any means, especially since an upstate New York sort of conservative is different from an Alabama sort of conservative. I think, in other words, that her conservativism (or moderateness, really), is in substantial part a matter of her personal philosophy rather than merely an attempt to position herself politically.
I also don't doubt that she'll be effective, compelling and popular, and may turn out to be a very good senator for New York. I just don't know that she'll be an especially good senator for Democrats.