I'm very interested to hear Christopher's thoughts on this topic - we're working on a project with the Faculty of Education here to develop an ePortfolio/reflection process (heavy on the ePortfolio side - likely using Pachyderm and Drupal - that we're just in the early stages of putting a proposal/demo together for).
Weblogs offers several key features that I believe can support a constructive, collaborative, reflective environment. For one, it's convenient. The medium supports self-expression and "voice." Collaboration and connectivity can be conducted efficiently especially interms of participants' time or place. You can access and link to a number of appropriate resources. It provides multiple communication channels (e.g., you can write, record and/or cast your thoughts). Publishing your thoughts online forces you to concretize your thoughts.
Collaborative weblogs promote the idea of learners as creators of knowledge, not merely consumers of information. A collaborative environment like the one I'm suggesting can allow peers to be seen as valuable sources of knowledge and ideas; a connection that participants can rely on beyond any formal classroom structure, i.e., collaboration leading to a community of interest.