I'm sure this has been discussed in other places, probably including meta SO, but I'm just going by my opinion:
The problem with tags in general is that the more you have, the less likely they will be used consistently.
Tagging in general can be confusing to people who aren't familiar with the SE platform, so keeping fairly obvious tags makes it an easier decision for them, and cuts down on the "clean-up" other users will have to perform.
Even people who are familiar with SE and tagging can find it confusing when coming to a new SE community and finding that they have a whole bunch of specific sub-tags that are expected to be used.
It's not going to be intuitively obvious that a Doctor Who question focused specifically on Amy Pond should be tagged with both doctor-who and amy-pond (or even doctor-who-companions).
The more tags you add, the more likely tags are to be missed, and the more likely a question will not be able to accommodate all of the tags that are relevant.
For example, let's ask a question about the conception of River Song and the involvement of the Silence. Now suppose we have all kinds of specific tags. There's a limit of 5 tags per question. So how do we tag it?
doctor-who amy-pond rory melody-pond tardis the-silence doctor-who-companions
Too many choices, and too many to fit on one question.
The only way tags serve a useful purpose is if they're used consistently. If some questions could have a tag, but don't (because no one noticed that the tag existed and was appropriate for the question, or because the question didn't have room for the tag due to other tags already in place), then the utility of being able to search, favorite, or ignore is broken.
The best way to do this is to limit tags for important distinctions, that directly support what people may reasonably be expected to want for facilitation of search, grouping, and ignoring. Part of this is admittedly semi-arbitrary, as one person may want to favorite all questions relating to a specific character (e.g. "I love Pinkie Pie from MLP!"), but few other people who participate in the community may be interested.
That's why it is best to bring specific instances to meta if there's even the slightest doubt. If the community decides there should be a pinkie-pie tag, then so be it, but if the community doesn't agree on it, chances are it won't be used consistently.