Oh, and some of the menu options have a tooltip (like "Date" and "Type"). It would be nice to mark these as such with an asterisk ("Date*").

E.g., if a user is looking for the field "Genre" (which is missing from the Advanced Search options but can be searched by selecting "Type"), the asterisks would give a hint that some menu options don't just represent a single field.

Oh, and I was trying to figure out what the "Search subfolders" checkbox does. It confused me, since I assumed that it would affect the scope of general searches, but apparently it only affects searches by the "Collection" property.

See also the old thread http://forums.zotero.org/discussion/2051/reports-without-the-notes/?Focus=8716#Comment_8716 in which Dan recommends renaming the option and disabling it when the "Collection" property isn't searched. As for the new name, maybe "Include items in subcollections" would be better?

Other than using asterisks (or some other string like " [+]"), the only other solution I can think of is using a hierarchical menulist, which seems to be possible using XBL: http://comments.gmane.org/gmane.comp.mozilla.devel.xpfe/5350

Not sure implementing such a complicated UI is worth the trouble though, although it would allow for more flexibility in advanced searches (e.g., you could make it possible for users to search by "Type", or by any of the Zotero fields mapped to "Type", such as "Letter Type"). My vote is to start with asterisks.

Jacek: I agree that the drop-down should better convey which mapped field belongs to which base field, but note that you're essentially asking for the list to be probably nearly twice its current length, and you're asking for there to be, for example, a "Website Title" field that wouldn't search, say, "Blog Title". So just "splitting" the fields doesn't make this unambiguously simpler or cleaner.

Also:

Search by journal title should be clearly named (there is only search by Journal abbreviation, to search by Journal title I think one needs to search by series title).

There's no "Journal Title" field in Zotero. The field in the Journal Article item type is "Publication", which is also the base field that shows in the search drop-down (mapped to various other type-specific fields that you can see by hovering over it).

In any case, Rintze's suggestion of a hierarchical menulist would probably be an improvement. That would still increase the length, but it'd make it clear which fields belonged to which base and let you search by either the base or the mapped field:

[...]
- Creator
  - Author
  - Editor
  - Contributor
  [...]
[...]
- Item Type
[...]
- Publication
  - Blog Title
  - Book Title
  - Dictionary Title
  - Encyclopedia Title
  - Forum/Listserv Title
  - Proceedings Title
  - Program Title
  - Publication
  - Website Title

The downside to this is that you probably never actually want to search for "Website Title" without also searching "Blog Title". It wouldn't force you to ever select that, of course, and you could easily find that and then select the parent "Publication" field, but having the individual fields be selectable would make things a bit more complicated.

Alternatively, it could just show those mapped fields in gray below the base field (not counting the creator types, which would be individually selectable). The downside to that is that you wouldn't be able to navigate to a particular mapped field by typing the beginning of its name, but most people don't know you can do that anyway. I think I'm generally in favor of this approach.

I've created an issue for this. If someone wants to work on this, they're quite welcome to.