definitions (descriptions) for fields as "bubble help"
I think Zotero would benefit from a bubble help that is displayed when the mouse hovers over a field name (to explain the semantics for the un-aware). The text to be displayed should be part of the locale files defining translations.
We also get surprisingly few questions on this, so it's not a major concern for most users (probably because fields are self-explanatory for standard item types, it's mostly unusual references that are unclear). For those that are more concerned with details, I'm thinking online documentation is probably a better choice.