Here is a recent talk I did at Cambridge University on think tanks - suggesting an evolutionary framework for categorizing and understanding different ones:

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During my talk, I tried to distinguish the roles of think tanks in asking new questions and opening up new ways of seeing, from their roles working within established paradigms and aiming at more incremental knowledge.  

The first group tend to be weak on evidence, and have to be; the latter group need to be much stronger than they are. But the broader challenge for think tanks in many fields is a sharper sense of epistemology - what kinds of knowledge are they trying to create or spread?  

In some fields, practical knowledge may well be ahead of theory - hence the spread of do-tanks which both learn and persuade through real life examples rather than pamphlets. I also wrote a short piece to accompany Demos' 20th birthday.