Other than the obvious (but surmountable) technical challenges with crawling and indexing, trying to establish "goodness" for a given user is tough. For a blogg

Other than the obvious (but surmountable) technical challenges with crawling and indexing, trying to establish "goodness" for a given user is tough. For a blogger it will be "hey, you are reading this so you probably like what I like". That's often true but as soon as you try to have a centralized service with arbitrary users, it is hard to do anything better than filtering purely commercial content.