"I'm curious," you wrote, "what you think might spark grander ideas to galvanize the left, other than an ambient sense of spiritual hollowness amid affluence." Good question--one for which I have no answer. That is, of course, partly an occupational hazard: It is really hard to put the words "grandeur" or "idealism" in the same sentence as the word "economist," except ironically. Uninspiring stuff is my business. (To be honest, I don't personally find the stuff uninspiring. One can find not only aesthetic pleasure but even a sense of mission in the effort to make sense of economic affairs: The Asian financial crisis is a terrible thing, but the effort to understand it--and to prevent a recurrence--is giving rise to some very exciting research, some of which may save the world. But these are essentially technical rather than moral issues, and therefore compelling only to technicians like myself.)
So is my sense that there aren't any grand ideas merely a reflection of my own narrowness? I don't think so. America is prosperous and at peace, its society imperfect in many ways but not grossly unjust or corrupt; unless we are prepared to take on the problems of less fortunate nations--and Kosovo aside we aren't--there are no compelling causes. That is not to say that I wish things were otherwise--you wouldn't want to conjure up a powerful external enemy in order to experience the moral equivalent of World War II, or re-create institutionalized racism in order to be able to have a second civil rights movement--but it does mean that there is a smallness and triviality about our current political life.
Incidentally, I don't think that the right is doing any better. As you said, the grand ideas have gone down in flames. Americans don't want a radically smaller government, or a return to 1950s morality. So the right, too, is at best fiddling at the margin. Indeed, there is something almost pitiful about the big-idea conservatives of yesteryear--whether it is the religious right trying to convince the public that we live in an age of dreadful immorality, or old supply-siders trying to remind people that the long expansion under Reagan (who? Don't you mean Clinton?) proves that cuts in income taxes are what America needs today.
What would change all this? Probably only really bad news. If the United States boom should turn into a Japanese-style bust--which is a possibility not to be dismissed--some of the big issues would be back on the table. That's not either wishful thinking or a prediction, by the way--just an observation.