So, the DPJ actually made it (I'm a perennial pessimist regarding that party). They seem to be fairly well prepared for a power handover, so we should expect to see them get to work relatively quickly, assembling a government by mid September. How much actual change will we see? for this first year, expect only easy, highly visible, symbolic changes. The reason, of course, being the upcoming upper house election - yes, we're doing a repeat performance of the hit political show of the year next summer - and the DPJ will need to show some kind of visible progress to the voters by then. Anything very complex or time consuming may well be started but probably not pushed until after that election. Also, any issues where the DPJ is at odds with coalition partners SDPJ and the PNP will wait until after the election, when their support might no longer be necessary.
As for the fundamental questions faced by the Japanese society - things like the gulf between salaried and temp workers; the low birthrate; the rampant inequality and paternalistic social systems; the underutilization of women in the workplace - the truth is that the DPJ really is in no better position to address them than the LDP was. It is a broadly conservative party when it comes to social issues and a substantial part of the party is aghast at the thought of changing tradition-bound systems even in the face of a slowly unfolding disaster. A number of other high-profile problems, such as social security systems that no longer fit the way people live their lives, are tangentially connected to these issues and thus unlikely to see more than temporary band-aids.
For instance, many laws and regulations assume and actively promote the 20-century family unit (actually a fairly recent family type, even though many conservatives seem to believe it's traditional) of one married breadwinner, one stay-at-home parent and their children. This life is proving to be impossible to achieve for many young people, since temp jobs are not stable or high-paying enough to marry and support a family, while two working parents are very discouraged by lack of day care, rules permitting (even expecting) workplaces to fire women once they get children and other disincentives. But changing rules so that two working parents are encouraged, making it easier to be a single parent, and making it easier for women to continue a career while having children, is anathema to many conservatives and seen as a path to moral dissolution and the destruction of society.
You can certainly argue that in this they are simply following the overall attitude of the country as a whole - and Japan is indeed very conservative with regard to some social issues. But in times of crisis a leader is expected to actually lead and drive opinion rather than follow it. I don't see the DPJ do more than baby steps in this direction, just to mollify its SDPJ partner, and I'm not optimistic that the DPJ would even be able to do much more without creating a serious, potential fatal rift within the party in the process.
The DPJ win is good, it is necessary, and it will hopefully cause some long overdue change in the governance of the country. But don't expect any actual long-term solutions to Japan's current ills from this government. The crisis will have to become rather more acute for anything to happen, no matter which government is in power.