I have recently noticed a large number of tag wiki excerpts which are not particularly useful. For example, the mathematics tag wiki excerpt said:
For questions about mathematics in English.
It seems that this could be easily deduced from the name of the tag alone, so I have changed it to be more descriptive:
For questions on the usage and meaning of mathematical terminology and the names for mathematical entities in English.
I have also noticed that tags are frequently misused, placing the burden of choosing the correct tags on the most dedicated users. I doubt many users bother to read the main tag wiki pages, so the excerpt is the only thing that will help most users determine if it is applicable (although in cases such as above the "excerpt" is usually the entire tag wiki).
If the tag wiki excerpts are improved, then the frequency of incorrectly tagged questions will probably decrease. I realize that nothing will correct this problem entirely, but I think better tag wiki excerpts would help users select more appropriate tags for their questions.