I think the most important would be the design which is the foundation for the course or module.
I believe that the programme should include extensive induction time before the online course commences. As a college, I am now putting this in place on my Masters Course, to increase the level of engagement. This will occur 2 weeks before the start of the semester and entry onto the course will only be allowed when the induction module has been completed which includes an online plagiarism test which must have a pass mark of over 80%.
The programme must also allow sufficient time for student dialogue and feedback along the lines of the 4 way conversational framework ( Laurillard 1998).
Within the design is of course the Constructive alignment which enables the learner to understand what he has to learn and why.
I think the frame work provided by the Communities of enquiries is probably the other important framework for enabling the online learning experience to be as effective as F2F, especially with respect to the teacher and social presence models which for me provided the scaffolding to enable this very complex task effectively.