Thanks to Shawn for finding this Outlook 2010 Instant Search bug.
When you use the individual navigation modules in Outlook 2010 and use search, the More button displays the fields associated with the folder type. For example, if you are in Notes module, you only see notes fields; in Tasks module, you see task fields; in Contacts module, you see contact fields. That's how it should be.
If, however, you use the Folder list, the More button only displays mail fields.
Because Notes (with limited fields) makes it so easy to notice the difference, I'm using screenshots from the Notes folder here, but it is a problem found in all folder types.
The solution, at least for now, is to use the navigation pane modules or type the field names in rather than picking from the More fields list. Type the field name (no spaces) in this format: fieldname:(keyword)
Once you learn the fields names, it's often faster to type the query anyway as switching between mouse and keyboard is slow. Use Ctrl+E to initiate the search and your fingers never need to leave the keyboard.
See my last tip for search query tips.