I notice that 3dbooru posts tend to have very few tags compared to danbooru. Is that because overtagging is discouraged here, or is it just that people haven't gotten around to adding more tags?

Are you talking my posts, nil's posts, or both?

Both/just generally. For example, there's no posts here tagged 2girls or from behind or large breasts, and only a handful tagged kneeling or wariza.
So I'm wondering whether it would be appropriate to start adding things like that.

Generally, I think danbo tends to lean a bit to the ridiculous at times but I don't mind tags like 2girls, 3girls, etc. I think it may be the only practical way to search for 2+ girl images without knowing specific model/character names.

On the other hand, I'm against "tag clutter" like positional tags. That being said, we are approaching the 300k mark so I'm open to discussion on additional tagging organization as long as it doesn't devolve into a spergfest.

I don't have a problem with tags for specific sitting styles as long as they're identifiably unique.

For my money, the counting tags are only worth a damn on images where there are unnamed original characters or gravure models.