It would be very useful to be able to watch a tag. This way a member can subscribe to discussions on content that interests the member. This would be a very powerful way to keep members interested, as they would receive alerts about content that interests them.
Such a very useful feature. Forums win in SEO on long-tail searches by bringing people together with a passionate interest in a subject. 80% of my Google referrals are from extremely specific threads where Google has decided that we are the authority. Being able to watch tags will help us to attract and retain members who create these authoritative threads.