Let's say I want to reference a member in a question. The member would probably be a significant contributor to the field or a known expert in the field relating to my specific question, so they might be interested in joining in on the problem to find an answer. Good for them, good for me. Can this be done currently in SO? Perhaps using the @ in front of their name.
There is potential for abuse and members could become annoyed by such tags. However, the notifications could be separate from the inbox, or filtered out if desired. This feature will appeal to members who recognize that a tag to an expert (with a large reputation) might not be as effective as a tag to an expert of a field with a much smaller scope. The feature will draw out those members and could further the richness of answers on SO. And the tags could be required to be added separate from the question (so as not to distract from the question itself), limited to only a few in number, and an initial rep requirement before the feature can be used.