I used to love AO3's tagging system more than anything. It's still a great feature, but not something I use as often anymore.
I've noticed over my time reading fanfiction on the site that the amount of tagging for a work really varies. I used to think it depended on the fandom whether an author decided to include many tags or just a couple in their work, but scrolling through my AO3 bookmarks page, I realized fandom wasn't really a factor in tagging. Each fandom has its own specific tags that tend to show up more often than others, but they don't really affect the number of tags.
I have complaints about authors both over-tagging and under-tagging, but when it comes time to post my own works, I find myself grasping for tags to include, so I understand the under-taggers.
But with works that are under-tagged, it's really up to the other fields to tell a reader what they are in for when committing to read a fic. One of my favorite Gendrya fics is about 150k words with a very short and fairly vague summary and almost no tags. The only real warning I was given that it wasn't going to be the happiest read was the "Major Character Death" archive warning, but within the "Game of Thrones" fandom, that warning can mean anything from canon deaths you expect to fic-specific deaths that will make you cry lots and lots. (That fic was the latter.)
I've also come across works that have no summary at all and very few or no tags, and I don't find myself wanting to read those. If I don't know what I'm getting into with a fic, I will pass it up for one that gives me more information up front. That's one of the things I like better about fanfiction than published novels: You are able to find a fic using tags and summaries and the other fields that will exactly fit what you feel like reading at any given time, rather than taking a chance on what a publisher includes on a book jacket.
If you don't include tags, how am I supposed to know if your fic will fill my need for a fluffy story with a happy ending? Or an angsty, adventure piece that makes me cry every chapter?
On the other hand, over-tagging can spoil a lot of the content in a fanfic.
While I am more likely to choose an over-tagged fic than one that is under-tagged, I still get annoyed when the tags take up a full desktop screen or more. If your tags and summary can't be viewed together without scrolling more than once, you have too many tags. Just saying.
I went through my bookmarks page to see if there were any examples of over-tagging, and I had to go back pretty far (I mean Larry fics bookmarked in 2014 far) to really explain what I mean about over-tagging. Larry authors had/have a tendency of tagging works like they were explaining the entire plot to a potential reader. There are a lot of unrecognized AO3 tags detailing the relationships between and backstories of a fic's characters.
This might just be me being snotty and annoying, but I don't need a tag saying "Harry lives in a London flat and Louis is his landlord" or "Liam and Niall are in a prank war" or "5 Seconds of Summer makes a cameo." Just let me read the fic and discover those things. Unless it's important that a reader knows a certain plot point or feature of the fic—like a trigger warning, information about what kind of AU it is, how angsty/fluffy they can expect it to be, etc.—it's better to just leave it out of the tags.
And while I'm complaining about over-tagging, that's not to say I won't read an over-tagged fic, but if I'm in a grumpy mood (it happens more than you would think), and come across one with tags like exampled above, it may peeve me enough that I form a vendetta against that fic and refuse to read it. At least, until I run out of other fics with that pairing.
I'm not saying you should tailor your tagging around what I think, or even really ever listen to me, but it couldn't hurt to consider your audience and the information they need to know about your fic before they read it while tagging.
How do you feel about AO3's tagging system? Are you an over-tagger or under-tagger? Do you have feelings about either type of tagger? Am I just being annoying and critical?