I have been introduced to the idea of a Threshold Concept a key stage in learning. The idea is for those who are attempting to teach to identify the Threshold Concepts in the problem domain and concentrate on them. I am working through some pedagogic material of considerable density. Does anyone have some references in this area for our domain here? -- John Fletcher
Glynis Cousin An introduction to threshold concepts www.et.kent.edu 
The most relevant page I can find here is Inorganic Complexity Threshold.
I have also found some criticism of the idea of Threshold Concepts including the following by Lane Wilkinson.
The search goes on.
I've been Doing Stuff to find out more about the Threshold Concept - This is the kind of stuff to be Archived To One Note -Donald Noyes.20140926
--- In the classroom - Andy Burkhardt:
From the page linkabove:
I recently read the new draft of the Framework for Information Literacy for Higher Education and really liked some of the threshold concepts. They felt more holistic than simply trying to teach skills piecemeal. I also was teaching a brand new class last week that wasn’t completely coming together for me — that is, until I tried using a threshold concept to frame the lesson. This seemed to make it click.
--- Professional Journal:
From description in the linked page above:
In this special issue the editors and contributing authors explore the idea that the field of educational development has threshold concepts: ideas about teaching and learning that have the power to transform the way educators understand the teaching and learning process and their role in it. J. H. F. Meyer and R. Land initially developed the threshold concept framework to describe aspects of students learning in economics in University settings in the United Kingdom. Since then, the framework has been applied to a wide range of disciplines, providing a heuristic for faculty to explore their own disciplines from a student perspective, and to use the results to enhance their teaching and their students’ learning.
--- Article:
Extracted from the above link:
Threshold concepts were held to be 'akin to a portal, opening up a new and previously inaccessible way of thinking about something' and were proposed to result in a 'transformed view of subject matter, subject landscape or even world view' (Meyer and Land 2003, pi). The proponents of a threshold concept framework generally refer to five key characteristics or attributes associated with threshold concepts. These commonly cited attributes are:
transformative
irreversible
integrative
troublesome
bounded
-- Meyer and Land 2005
Investigations of the viability of this framework in assisting teaching and learning have occurred across a range of disciplines, with interesting and primarily positive results. Although research into this emerging field is limited primarily to small scale studies, findings reflect that embedding threshold concepts into subjects can assist in student learning.
Examples of this include Kabo's and Baillie's (2009) article on social justice as a threshold concept in engineering;
Cope's and Staehr's (2008) work within the information systems;
Scheja's and Pettersson's (2010) study on threshold concepts in calculus;
Davies's and Mangan's (2008) work in the field of economics;
Kiley's (2009) application of the framework to doctoral candidates
Wow Thank you for the effort put in here. -- John Fletcher
Compare: Paradigm Shift
See also Wiki Concept, Wiki Concept Discussion
See original on c2.com