To add a few more things that should be worked in: 
 * Scalia and O'Connor: Between the 1989 Stanford case and this case, a mere four states changed their laws; ergo, saying that a "national consensus" that did not exist in 1989 has emerged since then is bordering on silly. 
 * Scalia: Pointing to a treaty that the U.S. has deliberately not ratified as evidence is suspect, as it seems to imply that the Court has the power to effectively ratify the treaty itself, or at least to bring its provisions into force. 
 * Scalia: The majority opinion claims that the Eighth Amendment is to be interpreted in the light of "evolving standards of decency", and also claims that the Court itself should consider in its own right whether a particular punishment is morally sound.  This, in Scalia's opinion, is akin to saying that the Eighth Amendment shall be interpreted in the light of "evolving standards of decency", and then defining "evolving standards of decency" to mean a show of hands among Supreme Court justices. 
 * Scalia: The Court's arguments at best show that, as was held in Stanford, there is a consensus to rarely execute people for murders committed while they were under the age of 18; not to never do so.  Therefore, deciding when it is appropriate to do so properly rests with juries, who are better equipped to judge particular cases. 
 * Scalia: Previous decisions that have overturned the death penalty in specific instances, such as for rape or for the legally insane, have required much stronger levels of national consensus than the ones exhibited in this case. 
 I think that covers most of the major points, although I may be missing something.