I wanted to post this for a while...
First off, I think that we're not tagging enough. Currently, we have an average of 2.1 tags per question. I've checked some (thematically) comparable sites, and Stack Overflow, Code Review and Programmers all have between 2.6 and 3 tags on average. Tags are a really useful concept of SE to organise the questions on the site and make it easier for people to find those they are interested in, and I think we should make use of that. I tend to do quite a lot of tag edits on new questions if I know that more applicable tags exist, but it would be nice if I could get some help with that.
In addition, while we've added a few tags for challenge types recently, it seems that some people put more effort into getting rid of tags than adding new ones. As far as I'm aware I'm one of very few people who regularly try to add new (non-challenge) tags.
At the same time, we do have a few tags that are completely useless, but still widely used. Luckily, we got rid of [algorithm] recently, but there's still stuff like printable-ascii, which I think is the epitome of a meta tag.
And then there's really confusing groups of tags, like math number arithmetic, where I'm never quite sure which ones to apply to any given maths-based challenge.
So before we embark on The Great Tagging Spree, I think it would be useful if we could get a consensus on what kinds of tags we actually want. The normal definition of a meta tag ("The tag can't stand on its own") does not really apply here, since challenge tags are mandatory. Can we come up with some guidelines for how non-challenge tags should be applied and which ones we should even create or keep?