Since Tags are mandatory to create a post, add a link to a page that'll display all available tags instead of making people guess blindly until they maybe stumble upon a tag that might be suitable to what they're saying appears.
Alternatively don't make tags mandatory.
I agree with @DSPatrick. The list of tags is in the tag list. We are working on adding description and other useful information on it. You can sort them by popularity or alphabetically.
If we don't make tags mandatory, experts on an area would not know what to tag. We are working on auto tagging to alleviate that problem and other improvements around tags.
