I think a knowledge aggregator fills a different purpose from a link aggregator. When someone has an idea they want to explore, they write something about it, and that can be put in a link aggregator. By default it's not yet ready for a knowledge aggregator. But it's more likely to become ready if people see it and discuss it.

I think transience is okay for a link aggregator.

Links can't be grouped, categorized.

Link flair isn't perfect, but it allows this.