How late are planes?

Another NYT piece: Ugly Airline Math: Planes Late, Fliers Even Later citing some work by MIT's Cynthia Barnhart on airline and passenger delay. Is this due to weather, demand (and high load factors removing slack in the system), or poor labor relations?

The article focuses on Northwest and Continental, though the observations are I am sure more general. The 25 minute average delay seems low to me, but I am operating off anecdote here.