PhD research questions

Last weeks I've been trying to concentrate on defining my research questions. It's not an easy task. Firstly, there are so many things to look at, and then once you start framing the questions, they all becomes slippery and unclear, redundant and such. However, this is my last take on it:

Main question: What are the main barriers to discovery, re-discovery and use learning objects and how can Social Information Retrieval help?

• Subquestion 1: Are there any specific requirements for the recommender system for LOs?
  Do we need to have any kind of “pedagogically aware” data or is it enough to use the existing models (such as movie, book recommenders) based on CF and content filtering methods?
  
  
• Subquestion 2: What is the role of explicit information input such as ratings, reviews and annotations to enhance the discovery and and use of LOs? Related to "classical" collaborative filtering using ratings, reviews. (these are the reviews that people do mainly to help other people)
  
  
• Subquestion 3: How can tags, lists and bookmarks enhance the discovery, recovery and use of LOs? e.g. “Playlist”, a feature in teachers' basket that allows them to make lists of LOs for a lesson, course, etc. “sequencing” This is stuff that people do for themselves, as part of PKM.
  
  
• Subquestion 4: When using social information retrieval (SIR) techniques, what are the differences between anonymous (such as classical collaborative filtering using ratings, reviews) vs. self-manifested, explicit social networks (e.g. Yahoo MyRank2) as input for a recommender system? How do users perceive these, are there preferences, what type of interactions, etc? What would be the hybrid model like?