What is the problem with fractions? How do we fix it?

Preparatory questions and links:

Focusing questions:
What types of fraction cause difficulty - vulgar, proper, improper, mixed number, ratios, percentages, decimal...?
When you or I talk about fractions, how do our definitions differ from our students'?
Apart from pizza, cake or pie, what other ways have you used to introduce or teach fractions?
When do fractions seem to become a problem - is it when they are used in arithmetic?
What concepts related to fractions seem particularly difficult to grasp?
What specific techniques do you use to clarify or teach fractions, other than 1/2, 1/3, 2/3, 1/4, 3/4...?
What approaches do you use when teaching conversion between fraction types - e.g. vulgar fraction to decimal fraction?

(Provided by @ColinTGraham)

Useful Links:
Preparing to teach fractions from The National Centre for Excellence In Mathematics Teaching (UK)
Departmental Workshops - Fractions Structured workshops for Professional Development (NCEMT - UK)

(Provided by @timstirrup )

Threaded discussion from 29th July 2010 -
Threaded discussion from 2nd August 2010 -

Links mentioned in the discussions, with contributor:

Resources (costing money!) mentioned in the discussions, with contributor:

Other links which may be useful