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.
replywhat you mean we can't? there are a lot of curated content directories out there.
replyRight, I suppose I mean "getting more people to think about why a few of these bookmarked for your favorite topics, especially tied to a trustworthy person, is a million times better than just hitting up Google."
replyOr, perhaps, a "a better Google should just take you to these."
Something like that.