I'd consider instead [what makes bad tags](http://blog.stackoverflow.com/2010/08/the-death-of-meta-tags/) to help answer that question. Ideally, tags are ways to help connect people with questions, not a way to classify the contents of a question. So, for example, ubuntu-desktop was a terrible tag. No one really wants to watch the ubuntu-desktop tag. In addition, the `bug`, `crash`, and `error` tags are horrible in that they're describing the content of the post, but no human will ever really want to follow the `bug` tag for one of two reasons. One, we're not a bug tracker. Two, it doesn't help convey any real meaning. However, things like specific software being used, [tag:wine], [tag:php], [tag:juju], [tag:ubuntu-one] make more sense as there's a chance people with knowledge of that software will follow. There are also a few additional tags, like [tag:application-development] which connect a subject matter (not a physical piece of software) with people who know and specifically work on application development.

Tagging isn't an easy thing, which is why it has a high reputation entry level for creating a new tag. If you're uncertain if a tag is good or not always feel free to post on meta. It's better to figure out early on if a tag is a good idea [before it becomes an unruly beast](http://meta.askubuntu.com/questions/879/is-the-ubuntu-desktop-tag-immortal)