While Cass Sunstein is right to caution against groupthink in the President's inner circle, I'd warn against holding up the Reagan administration as an ideal alternative. By all accounts--including those of celebrated Reagan biographer Lou Cannon, historian Sean Wilentz, and numerous contemporary observers--the Reagan administration was a dysfunctional nest of snakes.
Reagan's senior staff was ideologically heterodox, but Reagan was so hands-off--preferring to lay out broad principles, and then act as a salesman while his advisors took care of the details--that Haig, Meese, Baker, Deaver, Shultz, and the rest were constantly engaged in a battle royale to set policy on their own. They often ran to daddy as a tie-breaker--something that, after reading Jason Zengerle's piece, one could easily imagine happening in a McCain administration--but even that sometimes proved problematic, since Reagan hated seeing his staff quarrel. Hoping to please everybody, he would simply split the difference and combine random elements of each adviser's position into a logically incoherent whole.
As Sunstein notes, Reagan did manage to be decisive and effective on certain key issues (especially on taxes during his first few years--when James Baker was lording over the administrative process--and late in the administration, when Nancy Reagan was taking charge). But Reagan's non-management style also set the stage for years of painful deadlock, leaks, and backbiting on multiple (often deadly) fronts--as well as blunders like Iran-Contra, where members of the administration felt that they could effectively freelance American foreign policy.
Ever since Yale psychologist Irving Janis invented the term "groupthink" using the Kennedy administration as his initial case study, executive branch-watchers have obsessively warned against letting this type of think in to the Oval Office. Groupthink can be debilitating, but so can other, less-famous genres of management failure.
If Sunstein's end goal is to render an administration more pragmatic and less extreme, then choosing Reagan's style--with all its drawbacks--is not the most direct way to go about it. You could get the same positive results (indeed, better ones) by simply filling an administration with pragmatic, reasonable people who answer to a president with excellent judgment--then running it like a well-oiled machine.