One of his points will be controversial to some people, I think: Every contributor to the project tries to take part in the interface design, regardless of how little they know about the subject. And once you have more than one designer, you get inconsistency, both in vision and in detail. The quality of an interface design is inversely proportional to the number of designers.
I don't think this is necessarily true in a non-opensource environment; and, to be more concrete, in a software environment where people aren't argumentative prima donnas, communicate regularly, and reach consensus before implementation of the crucial features. But when there's frequent handoff of work, a tendency towards grandstanding or power plays in the design phase, or poor communication, it will be true.
Updated to add: He has a sequel article based on comments he got on the original, at Why Free Software usability tends to suck even more. His points continue to be good, including the inspiring last comment, which I think is also is true in any organization: As with previous critiques of Free Software, each of these weaknesses will become less of an issue proportionally to the number of contributors who read about them, and learn to recognize and combat them.
In software companies, this is known as "risk management." Doing that well in a design process requires recognizing the failure modes, worrying about them, and making yourself immune to them.