Sylvia,
You mention an essential element in the decision to use synchronous or asynchronous - quality.
A boring lecture is still boring whether it's synchronous online or f2f. I still think, though, that there are sometimes compelling reasons to use syncronous content delivery. For example, I've worked with several instructors who communicate verbally much better than in writing. Interactive, syncronous voice systems like Skype or eLive can help draw out these stengths.
Given this, I'd have to say that most in-depth content-based discussions are better done asynchronously, and I'd generally reserve synchronous interaction for topics such as class introductions, meeting the teachers, or discussions about process or project management.
Nevertheless, just last week I recommended to an instructor that she use a weekly or bi-weekly eLive session for a Practicum course. Part of this decision was based on the instructor's excitement about this technology. The learners will be all over the world with each of them working in a different placement. They will rotate sharing their placement experiences. I'm hoping that they'll get a sense of connection, support and encouragement, and a sense that there is a community backing them. I'd welcome more ideas about how to achieve these goals more effectively!
My goodness, these choices are complex!
Vivian