Japan: basically the nuclear problem is Three Mile Island. As we saw there in hindsight, the problem was mostly human psychology and not human physiology, much less nuclear physics. The real problem in Japan is the humanitarian crisis brought on by a tsunami, a massive earthquake, the problem of missing families and dead, destroyed property and economic (and human) value, and this is exacerbated by humans being moved around unnecessarily because of radiation fears that aren't quite warranted. Maybe it helps psychologically, but really it just terrorizes a people who have actually had a real radiation related history (ie, WW2's dramatic conclusion).
Also: there's a huge "what happens to us as America" problem. "NOT EVERYTHING IN THE WORLD IS ABOUT YOU!" is my reaction to this. Stop being a complete arrogant prick and start worrying about other people is the advice I'd offer Americans. I've seen worse impulses in our reactions to this (things like, eh, Japan's had a nuclear bomb dropped on it before, they'll be fine), but the fact that we're so focused on the nuclear situation and possible implications, what few there are, for nuclear power in America means we're avoiding dealing with the real world implications for real Japanese people who could use our assistance and concern. The actual impact of the Japanese disaster on America is mostly long-term economic trade and the psychological fear dredged up by radiation and nuclear power that goes back decades and has little or no real world evidence to support its existence. These are not things that should actually concern Americans, at least not in the present term. People who are suffering and require food, water, medicine, and shelter should.
Libya: I'm sure we want to signal our support for a humanitarian concern brought on by the slaughter of innocent civilians by a brutal dictatorship trying to retain control of "his" country. And to the extent that this is a reason for intervention, I'm fine with that. This is not the reason for intervention and more importantly, this is not the outcome of such interventions. Pointing us at Bosnia and Kosovo and Somalia doesn't engender a likelihood of successful results with a less repressive regime being installed instead and the general welfare of the people of a nation lifted by military force raised in their defence. Pointing us further at Iraq parallels really worries me. Yes we are agreed that Qaddafi is a bad man and should not have been entrusted with the governance of a country. What next? What do we propose to do about this? If there isn't an answer to that question, we should not be willing to even half-ass the situation and fight a war intervening on any side of the conflict. Maybe the justification is that we don't want another Rwanda-type humanitarian crisis that we "could have prevented" in a country we have somewhat more international strategic interests in (ie, Libya's oil). I'm not sure, and certainly not convinced that we could have prevented Rwanda. Maybe we could have simmered it and spread out the violence over more time, maybe more alcoves of survival would have been available. But we really don't have much ability to run into a country and install institutions that will grant, much less guarantee, not only stability and safety but a liberal pluralistic democracy in a tribalistic region. Again: see Bosnia, Somalia, and Kosovo. None of these is particularly better off. In most cases all support for one side did is allow that side to repress and slaughter the people who were repressing and slaughtering them prior to our intervention.
And in any case, the most pressing reasons I'm finding for the average American on the street to wish to intervene are to watch things blow up on TV. This has little to do with humanitarian goals. This isn't their fault so much. They just don't know dick about Libya, Libya's current political and military situation, and don't care to know. Things go boom, I'll watch seems to be the thought process.