I put a bit more thinking in this feature and use case. Perhaps the best is not "follow tag" but a rule for "auto follows discussions that are in a given tag".
The problem with "follow a tag" approach is that you follow all the discussion and have no way to unfollow specific discussion that have the given tag
I think it would be easier and more flexible to have a system that automatically make you a follower of discussion depending on some kind of rules (i.e. author of the tread, the role of the author keyword and tags). Then a UI for this specific use case (follow tag) can be built on top of that. It should also have a way to unfollow all discussion based on some rules (perhaps search in enough -> unsubscribe all discussion in the search result).
With such system, you can then practically follow all discussion of a given tag, but you can still unfollow specific discussions you don't want to follow anymore. It's also a flexible system for all sorts of plugins.
