• certification - this had some value some time ago... What is the current state of the Ubuntu Professional certification? and now people uses it instead of certificates
• tutorial - in my eyes they seems as useful as learn
• training - odd questions, too vague, too broad, too bad... this tag seems related to more odd tags
• documentation - misused, the tag is there for questions asking about the documentation as whole (writing, managing, distributing, creating documentation), but people seems to use it to ask for some kind of documentation which is suboptimal (there should be a question about where is all the documentation Ubuntu provides... I remember answering one of that kind, which also has the documentation tag)
• web - D: This one seems about anything. Let me repeat, this tag seems about ANYTHING. From PHP to web servers to "this web page doesn't run in Ubuntu?". Is way too broad while it can be break down in smaller, more concise tags.

So, the mission is:

1. If the tag is about a topic, and the topic is on topic. Then create an excerpt/wiki and make sure all the questions are correctly tagged according to the excerpt.
2. If the tag is about a topic and the topic is off-topic, too broad or otherwise lead to bad questions, remove it. I remember that some user used the presence of a tag to infer that his question is on-topic.
3. Tag is too vague, remove it.
4. Another tag describes better the question, retag it.
5. Not sure how to tag a question, ask in chat.
6. If you are a less than 2000 reputation user and see any bad tagged question that needs also improvements, just edit it. Reviewers will evaluate if you edit was useful, but be careful about spamming reviews ;).