What is a better model for using tags on social content websites? Tag Cloud? - Since this caters to the long tail of users looking for 'interesting' content?

What is a better model for using tags on social content websites? Tag Cloud? - Since this caters to the long tail of users looking for 'interesting' content? eg: flickr Category tree? - Since this gives more targeted/specific information to users 'looking' for specific information. The categories again could be designed through user generated tags. eg: subreddits, digg Combination? - Is the 'best of both' a good model? Or, will a tag-search based model suffice? Or, does it depend on how domain specific the social content platform is?