As a law professor, I think there is a natural inclination to quibble with every Supreme Court opinion even if you agree with the outcome. Here, I agree with the outcome, but I also think Kennedy's opinion is quite good. My opinion, however, has one notable caveat. Kennedy's opinion is quite good insofar as it utilizes and applies existing 8th Amendment jurisprudence. However, I think such case law is prone to all sorts of problems (some of which are discussed in the dissent).
Nonetheless, I think Kennedy gets the better end of most of the arguments with Alito. Is there a national trend? No. Kennedy successfully shows no such trend exists. Alito's point that Coker prevented such a trend is fairly well rebutted by Kennedy. I think Kennedy could have gone even further because Alito provides no legislative evidence to support his argument (Alito cited only a few court opinions that interpreted Coker broadly in other situations). Alito is making bold statements on this point that just aren't supported by evidence.
What about the subjective justification for the application of the death penalty? Here, I think Alito does much better. This is always a strong argument for advocates of capital punishment in such cases. However, again Alito plays fast and loose with some social science and doesn't do that good of a job explaining why child rape is that much worse than the rape of an adult. Again, I think the dissent would have been more coherent if it fully embraced and discussed an originalist interpretation and supported overruling Coker. As it is now, the dissent seems contradictory in several places. Still, I'd give Alito a slight win on this issue, but I'm not sure how this win even fits into the dissent's methodology.
On the policy arguments, I'm clearly biased because the Court cited my article in support of one of the major policy objections to the Louisiana statute. However, I think Kennedy does a much better job discussing why these laws may not serve punishment goals. Alito is at his best when he says that such considerations should be irrelevant. Again, though, this argument would be much stronger if Alito would fully embrace an originalist idea. As it is, his argument is inconsistent with a lot of prior case law since punishment goals are always evaluated in 8th Amendment cases.
The narrowing issue which had some post-argument buzz has almost completely disappeared. It is addressed in relation to the main argument rather than as a separate issue. There is no real discussion on the argument at all.
So, in the end, I'm quite happy with the way Kennedy applied the law even though I think the Court might be best served by going in a different methodological direction.