This post is simply a list, to be added to, of research papers evidencing factors common to all types of counselling and psychotherapy which are cited as influencing treatment outcomes. Please feel free to add suggestions!
Norcross & Wampold: discussion on Drug & Alcohol Findings (website).
Zuroff, D.C. & Blatt, S.J. (2006). The therapeutic relationship in the brief treatment of depression: contributions to clinical improvement and enhanced adaptive capacities. J Consult Clin Psychol. 74, 130-40.