John Socha-Leialoha wrote up a nice bit of insight on how Users are Idiomatic. John writes:
"First, different users will have different definitions of "intuitive." ... Second, and this isn't conveyed directly by the definition of idiomatic, users actually expect inconsistent behavior."
In my experience, I've found this to be true (user experience walkthroughs with customers are very revealing and insightful).
I first got introduced to idiomatic design for user experience several years back. One of my colleagues challenged me to improve my user interface design by trading what might seem like intuitive paradigms for more useful idioms. He used the example of a car. He said the placement of the gas/break pedals was not intuitive, but idiomatic.
He argued that what's important is that the pedals are placed where they are efficient and effective, not necessarily intuitive. His point was that I should make design decisions by thinking through user effectiveness/efficiency in the long term vs. just thinking of up front discoverability of intuitive models. He added that sometimes intuitive placement makes sense, as long as you're not trading overall user experience.
User experience in software is challenging so I enjoy distinctions like this that make me think of the solution from different angles.