Adbusters has an interesting article on everything wrong with liberalism in US politics today. It raises many interesting points and (spoiler alert) ends with adopting progressive as the appropriate moniker for the American left.
Progressive seemed to me a re-branding term, and not as worthy as re-claiming liberal, but the power dynamic difference makes sense. 'Liberal' has been adopted into the 'pinko-commie' category as a style of resistance to those in power. Liberals are the changing of the guard, and so can never be the guard. Once in power and kept in power, the resistance to established authority is weak, as by being in power, the elected liberal is now a member of the authority.
Progressive is used, instead, as a way of moving forward, of working for positive change, rather than fighting against power. It is goal oriented, and not defined entirely by what it is in opposition to.
As a side-note relevant to the title of the post, the article has the 'lets not let the baby-boomers define us again' attitude that seems to be cropping up everywhere. Christine, in the depths of her archive, spotlighted a debate between the Gen-Xer minister and the Baby Boomer Minister, and the Adbusters article has a similar ethos to it - "yes, the sixties were great, yes, a lot was accomplished. The sixties are not now, and while it doesn't hurt us to have a sense of the history of our cause, we must acknowledge the differences from now and then, and we must be something different than we were then."