Is it bad form to use the <abbr> tag to explain words that are not actually abbreviations but to produce a "hover over" explanation of content?
If it is, why is it bad form, and what is a good HTML alternative?
You actually don't need to use the <abbr> tag to use the title= attribute. You can apply it to many things, including <span> tags.
Example of use:
<span title="This is my explanation here.">Confusing text</span>
From w3schools.com: By marking up abbreviations you can give useful information to browsers, spell checkers, translation systems and search-engine indexers.
In other words, you'll provide misleading information to search engines when there is no reason to, by incorrectly using the <abbr> tag.