Since I started the #worldbuilding tag, I check it occasionally to see if there's any new posts in it. An alert might be neat, but I probably wouldn't use it. What might work better would be to be able to "super subscribe" to tags/people/sites, so when a post is made using that tag/by that person/from that site, your little notification wheel lights up with that post title inside.
Not a bad idea, although it seems a bit redundant in that your feed is supposed to serve that purpose. What do you think of the idea of being able to sort your feed into tag-only, user-only, or domain-only content? Another approach might be to select some tags to 'autopin' in your feed.
Ah, but the normal feed throws everything at you at once, and posts jump around and rise back to the top and get all kinds of crazy. The Global feed works strictly on time (I think), but you still might miss a bit if you haven't been on in a while and there's pages of new stuff to sort through. The "super subscribe"-type option would be static in your notification box until you dismissed it.