Where can I find information about what would make a beneficial tag on Ask Ubuntu or any other Stack Exchange site? What conditions must be satisfied before a new tag should be proposed/made? For instance, I think that Ask Ubuntu Meta should have a "Criteria" tag. This tag could be used for questions (like this question) asking about the criteria for a certain subject/topic.
I'd consider instead what makes bad tags to help answer that question. Ideally, tags are ways to help connect people with questions, not a way to classify the context of a question. So, for example, ubuntu-desktop was a terrible tag. No one really wants to watch the ubuntu-desktop tag. Yes ultimately questions tagged about this revolve around the ubuntu-desktop, but it's nothing pertinent to the content of the question being asked. It's simply supporting information.
In addition, the
error tags are horrible in that they're describing the context 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. So, the fact that you're saying it's a crash means nothing. The context of why you're asking the question isn't as important as to what you're having problems with.
However, things like specific software being used, wine, php, juju, 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 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
I actually re-read your question and noticed this.
For instance, I think that Ask Ubuntu Meta should have a "Criteria" tag. This tag could be used for questions (like this question) asking about the criteria for a certain subject/topic.
I would argue criteria is a "meta tag" (bad thing). It doesn't offer any additional clarity on what the question is about. The question is a discussion about tagging. The fact that you're asking for criteria is irrelevant. If we were using tags to catalog questions, then yes criteria would make an excellent tag. However, tags aren't about cataloging but for humans to use. Criteria is meta in that it only offers context of the question, but does not necessarily clarify the content. I'd consider this also to be tagged faq as it's quite a commonly asked question.
The rule of thumb I use: "Would someone ever want to follow this tag?" If I can't think of a case where someone would want to follow it, then it's probably not going to be a good tag. Even if I can, I try to argue against it until I can't think of a reason not to create the tag.