Changes in depressive disorders from DSM-IV to DSM-V are listed here:
Diagnostic Criteria for Major Depressive Disorder and Depressive Episodes. http://www.psnpaloalto.com/wp/wp-content/uploads/2010/12/Depression-Diagnostic-Criteria-and-Severity-Rating.pdf
Pain-related issues are given in this paper:
Journal of Addictive Diseases Volume 30, Issue 3, 2011. Prevalence of Prescription Opioid-Use Disorder Among Chronic Pain Patients: Comparison of the DSM-5 vs. DSM-4 Diagnostic Criteria. DOI:10.1080/10550887.2011.581961
A big change is that there is a depressive disorders chapter. The mood disorders chapter was split with bipolar disorders going to a separate chapter. For MDE itself, the removal of the bereavement exclusion is the biggest change. The actual syndromal definition (5 of 9 A Criteria, at least one of which must be depressed mood or loss of interest) has not changed at all. The bereavement exclusion's removal attracted a lot of discussion, but in my opinion is not a major change in practice or research. Another change is that dysthymic disorder is gone and is now combined with chronic major depression (>2 years) in a new category called Persistent Depressive Disorder.