Tags would be much more useful if you implemented autocomplete on them.
So when you are searching and enter a has it would give a list of tags. This should reduce as more characters entered. Something like this would also be useful when adding tags to documents. Every other tool that makes use of tags has these simple features.
Without this it is easy to have many variations of a tag that means the same thing, making searching diffucult. Also when searching you are forced to remember what tags you have used.