Why is there no tag for metaballs? Metaballs are a matter of discussion like anything else, and a very distinctive topic.
While you couldn't yet create your own tags, I thought it would be useful to point out that there is a privilege earned at 1500 reputation Create New Tags.
Anyone who has 1500+ reputation can create a tag simply by
Enter[ing] a new tag with your question and it will be created. When creating your new tag, bear in mind that tags ...
- must be no longer than 35 characters
- must use the character set a-z 0-9 + # - .
- on some sites, new tags will be automatically culled and removed from the system if they are not used by at least 1 other question in a 6 month period.
- meta tags, tags that cannot stand alone as the only tag on a question, are not allowed.
In addition to creating a tag, you will create a tag wiki entry that is initially blank. Anyone can propose an edit to a tag wiki, but you have to have a reputation of 20,000 or more to approve such an edit.
This answer contains a good discussion on how to edit a tag wiki entry.