He gives six reasons, the most interesting of which--to me, anyway--was the second:
Capital juries are likely to be less sympathetic to the defense because they are death-qualified (i.e., only people who are willing to impose death are permitted to be jurors in a capital case.) This eliminates a not insignificant portion of the population that is most attractive to the defense.
I had never thought of this before, yet now it strikes me as a significant problem--especially depending on what the exact definition of "death-qualified" is.
For example, I don't have any theoretical or philosophical objection to the death penalty. But I do think that our death penalty system is broken beyond repair, at least without a moratorium. So, as a juror, would I be willing to cast a vote for death? Maybe...but I think I'd set my standard so high that it'd be impossible to meet--it would hinge on me being absolutely certain ("beyond a scintella of a doubt") of the defendants guilt, on it meeting certain admittedly subjective criteria for severity (i.e., a rape/murder), and being convinced that there was no procedural defects apparent at trial--ranging from inadequate representation to racial bias in the DA's office. I'm not sure I'd ever be convinced of all that.
People like me, then, probably will be excluded from hearing capital cases. Which is problematic, because what makes someone like me "like me" is that we're skeptical of the state when it comes to the criminal justice sphere. I really think that given the statistics, such skepticism is warranted and those who don't have it are really demonstrating a bias towards the accused (aka, the not-yet-convicted). Even if one doesn't think that we're indisputably right in our critique, though, it still should be recognized that our presence is valuable for a balanced jury pool that will be through and complete in its deliberations.
Incidentally, this post topic dovetails nicely with Andy Leipold's findings that judges are--contra to the prevailing wisdom--less likely to convict criminal defendants than juries are. This occurs off of all cases, not just capital ones, but still, I think there may be a connection.