In what way was the Law of Moses "a good thing whose purpose had now been accomplished"? I came across this quote from NT Wright:


  The Torah [the Law of Moses] is given for a specific period of time,
  and is then set aside—not because it was a bad thing now happily
  abolished, but because it was a good thing whose purpose had now been
  accomplished.


I'd like to see this fleshed out in more detail. In what way is this true, according to Wright or others that hold this view?
The question here really is "what is the purpose of the Torah?" and "what is achieved by God in Christ?" For NT Wright, they are in a sense the same. The key theme, as in much of Wright's work, is that of covenant relationship.

The purpose of the Torah, for Wright, is to mediate the covenant between YHWH and Israel. As Wright writes in Creation and Covenant,


  Torah does in human life what the sun does within creation: it brings the light, power and searching, probing heat of YHWH’s presence into the depths of the human heart. Torah is, of course, the covenant charter of Israel, the Law given to bind Israel to YHWH, to establish the nation as his people. With Torah as its guide, Israel is the unique, chosen people of the one creator God.


So the purpose of the Torah is to bind YHWH and hoods people together, to mark them out as his covenant people. Basically it is to mediate the relationship.

For Wright, one aspect of Jesus' work (perhaps even the central aspect) is again to mediate the relationship between God and his people of the New Covenant. The Torah mediated the Old Covenant, but that purpose has passed away with the coming off Christ into the world, the perfect mediation of the covenant of love between God and his world.