Back when the medium of ideas was physical, curators not only made choices about which items to include in the limited physical space afforded them, they created a culture based around works very purposefully shared. They thought through what was needed for a group, and what was worthy of that group. Thus did the limitation of the physical turn into a positive: Collections were shared in order to enable a group to have a common basis. This necessarily entailed choosing some works as more valuable than others, a decision that reinforced itself since the works chosen were then taken as a fundamental to the discipline or society itself.
The rise of the digital is changing just about everything about curation, mainly for the better but not entirely.
First, collections themselves used to be physical assemblages of works. Now, not only are the works unassembled, the collection consists of metadata about the works. The metadata includes not only where the object exists (usually a clickable address), but also information designed to help the user evaluate whether it's worth the click.
Second, because a collection is now metadata-a playlist-there is little incentive to stop at just one collection. This allows curators to speak in many voices, or to assemble a collection that represents many curators, just as your local bookstore may have "my favorites" lists of recommendations from each of its employees. This abrades one of the chief values of analog curation: the authority implicit in a curated collection. When you can only have a single collection, the works selected carry extra weight. Of course, that was an illusory sense, a fact that curators themselves generally recognized. A library acquisitions committee knows that it's making decisions that along the edges are arbitrary: You know you have to include the standard reference works, but for most of the works there's no right answer and probably no uniform agreement among the curators themselves. Now every curator can have her own digital collection even if other curators disagree.
Third, analog collections served as an implicit bounding of the responsibility for knowledge. There was something comforting about having a collection of Great Books curated by Mortimer Adler; read those and you've read your culture's Great Books. Likewise, if you're a student, your school's library represented the field from which you were able to choose—although more advanced students would go the extra mile and order through inter-library loan. With a curator taking responsibility, mastery was possible even if difficult. (I dare you to read all 400 Great Books. I double dare you!)
Fourth, analog curation made it feasible to read the works brought into the collection, leaving the rest difficult or expensive to get to. Digital curation often only brings an item to our attention and reduces the number of clicks to get to it. The items outside the collection are still available on the Web and may show up at the top of a search results page or on someone else's curated list. The cost is in discovering the item; once discovered, items generally are only one click away.
Finally, curation protects us from works that are a waste of time, works that would mislead us or works that are objectionable. In a digital world, we have lots of other ways of accomplishing these goals: We use recommendation systems of various sorts, and a wide variety of evaluative tools have emerged to help us decide what is helpful and what is misleading. But the very ease with which one can put together a digital collection is removing the sense of safety we used to have. You probably feel safe leaving your child at your local public library for the afternoon. Not only is the environment physically safe, you can feel confident that the books inappropriate to a child are either on a high shelf or behind a gate of some sort. If your child is young, you'll feel quite secure dropping her off in the children's reading room. But how many of us would be happy about dropping our kid off in the middle of the Web, or even at a crowd-curated collection on a site? We've all been unhappily surprised at what lurks behind an innocent subject line.
Curation is thus changing at its core. It's curating metadata, not primary materials. Multiple curations can exist in the same space. We are losing the sense that there is a right curation for almost anything, and are also losing our sense of mastery of topics. And collections often are not as safe as they once were. Because of its strengths, curation will be with us forever. Indeed, as the welter of content continues to increase, we'll have more of it than ever. In some areas—medical information, legal text—it will retain its old virtue of providing a reliable, authoritative source. In most areas, though, it has already been transformed, simultaneously transforming our idea of what constitutes a topic, what constitutes expertise, what constitutes authority and what constitutes a collection.