Here are some things that I suspect make tagging on SE work well:

• Searching by tags works well
• You can mark some tags as favorites so that they are prioritized or excluded for you
• Tags prominently displayed
• Tag suggestions based on commonly used tags/words used in your post : seems like a good idea
• Lots of people can retag posts : seems like a good idea
• Tag wiki pages: good idea, probably would not take much work
• Information about 'related tags'
• Could just be that there are so many disparate topics that tags are more important