What are the biggest challenges to formative assessment?

In the hope of widening my understanding of the perception of formative assessment, or assessment for learning as some like to call it, and also the broad nature of the challenges we face in the classroom, I have started this collaborative document.

People from all over the world have outlined what they believe are the biggest challenges to implementing and sustaining systems or habits of formative assessment. I will be looking for trends within the issues raised and hope to write a post soon about some of the common themes identified within it.

Please consider adding your own contribution to the document by editing it here and leave a comment of anything you are beginning to notice once you have read through the ideas so far.