I agree with you that there is little that can be changed when it comes to domestic policy, and that the DPJ is only offering so much, but I think there is something to be said for how the DPJ might make a real difference in Japanese foreign policy. A lot of the DPJ's past rhetoric has been very critical of the US-Japan alliance, and this may be one area where the DPJ can actually change the status quo.
The rise of China as a peer but not necessarily a direct competitor also means that the US may not have the same strategic view of Japan as a country to contain China as it once did, they may prefer to engage China rather then try and use Japan as a strategic hedge against them.
Considering the DPJ has had no foreign policy experience, this allows for even greater mismanagement.
I left out foreign policy in my previous post, in part because the thing was getting way too long already, and partly because I really don't know my around the field. But lack of expertise doesn't stop anybody else from bloviating about it. Why should it stop me, right?
First, as far as the election is concerned foreign policy simply doesn't matter. Election politics is local, and you don't get more non-local than foreign policy. You could put an entire Osaka bar to sleep by just mumbling "bilateral security agreement" at them. As long as a party doesn't come up with something utterly ridiculous ("Let's have an American base on the east side of Shikoku, a North Korean base on the west side and let them fight it out on reality TV!") it's just not going to be a factor in the election no matter how much the punditry is wringing their hands over it. This is nothing specific for Japan, by the way; unless you have a shooting war going on, foreign policy is rarely a factor in elections.
Second, I have no concerns at all about the foreign policy competence of the DPJ. The DPJ is partly old LDP members; the party itself has never been in power, but plenty of its members have. This is really just another variation of the "only those in power can be entrusted with power" argument. Would that argument hold water, Swedish foreign policy should have become a complete mess when a center-right coalition took power for the first time after over 40 years of Social Democrat rule. We did get an economic mess, partly due to inherited problems and partly because the coalition was unstable and unsustainable; but foreign policy, notably, was never an issue.
There is nothing actually wrong I can see about the DPJ stance on foreign policy. One may or may not agree with all of it, but just because we disagree with some idea doesn't necessarily mean it's wrongheaded. It just means the means for achieving common goals - security and stability - are somewhat different than what you prefer. Foreign policy is very much one of those areas where rational, well-informed people can reasonably disagree.
We may agree or disagree, but one thing the DPJ is absolutely correct about is that an open debate about the future of Japan's security arrangements is long overdue. It may well be that the very tight security arrangement vis a vis USA has run its course; neither country needs such a tight arrangement anymore, and the negatives start outweighing the benefits. As you say, China plays partly different roles for Japan and USA - naturally, since Japan is part of the region while the US is not - and that means that the security concerns as far as China are also different. That is an argument in favor of a looser alliance, rather than a tighter one.
Ozawa did spell out the situation quite accurately, for all the criticism he got: Japan taking on greater responsibility (in manpower, money and so on) for the security arrangement goes hand in hand with Japan being a more independent party in the arrangement. Can't have one without the other.
This goes for both sides. Some security hawks may want Japan to shoulder greater responsibility for regional security, but that will necessarily mean that Japan becomes more independent of the US in security decisions too; since the goals and the concerns are not identical, that will lead to more disagreement and occasionally directly opposing goals. Some people want Japan to get out from under the US and become more independent (this seems to be a goal uniting parts of both the extreme left and extreme right - for very different reasons of course), but that would mean having to pay their own way (in money, manpower and risk) to a whole different degree as well.
Foreign policy is a bit special in that it's both long term, and very interdependent, and the US-Japan security arrangement is an example of this. Changing it takes a lot of effort since you need to change a lot of specific policies in tandem; and the long term nature means you can't do it on your own; you need to have not just your current cabinet on board, but your entire party, your coalition partners, at least part of your political opposition (for long-term stability) and your foreign counterparts. Things just don't change fast or on a whim - which is one reason it doesn't make for good election campaign material.
With the exception of fringe fruitcakes like the Happiness Realization Party or the black speaker-van crowd, I don't see any Japanese parties being actively harmful to Japan as far as foreign policy goes. Just as I wrote about policy in general in my last post, the rhetoric may be strident but the reality is a bland compromise informed by the winners' guiding philosophy no matter what the electoral outcome.
You could even have the JCP or the social democrats in power and still nothing disastrous would come of it. For all demonization, people are generally not incompetent or actively stupid, and everybody has a shared interest in keeping Japan safe and on good terms with the international community. The means may differ as may the precise goals, but remember the bit about "reasonable disagreement" above.
Let's just for arguments sake assume that the unthinkable happens and a coalition of the JCP and social democrats come into power. Disaster? No. They would want to break the alliance with the US. But, not being insane, they would not try to kick everybody out then and there; there's ongoing treaties and contracts to honor and Japanese forces would be completely unprepared to take over any responsibilities anyway. What would happen in practice would be a phased out shift over, oh, 20 years or so, with responsibilities and capability gradually shifting to Japan over time. And it would all happen with the consent and active participation of the US - neither side would want to have the relationship deteriorate and would work to keep the relationship amiable even as the terms of it change.
In short, if there's any one area that really doesn't matter at all for this election, foreign policy is it. The electorate, by and large, doesn't care; there'll be no significant change in the short term no matter who wins; and any long-term changes will be consensual and so slow all parties will easily adapt to it anyhow.