I finally got around to watching the video of the tech talk that Linus gave at Google discussing the design of Git.
In this video, Linus explains a lot of the advantages of using a distributed system. But it is also enlightening because it's a window into Linus's motivations: he discusses the ways in which his own needs— as a system maintainer— drove the design of the system, in particular in the areas of workflow, speed, and data integrity.
One interesting idea is that in DVCS, the preferred development workflow (you pull from a small group of people you trust, who in turn pull from people they trust...) mirrors the way humans are wired to think about social situations. You cannot directly trust a huge group of people, but you can transitively trust many people via a web of trust— a familiar concept from security. A centralized system cannot scale because there are n2 pairs of conflicts waiting to happen, and they will happen, because groups of people are distributed (not everyone is in the same room at the same time on the same LAN). But a DVCS workflow can scale, because it is fundamentally based on interactions between people and not on the artificial technical requirement that there has to be a single canonical place for everything.
Warning: Linus has strong opinions. I think he refers to at least three different groups of people as "ugly and stupid" in the course of his 70-minute talk.