You can automatically export every single field as recognised by MC. No need to select fields for export...
1) Create a new playlist
2) Go to a library view where all of your items are visible. They can be grouped---that's OK. (e.g. On my screen, I see piles of album covers representing each artist.) Click somewhere inside the window so that the window is activated. Press "Ctrl" and "a" (for select all). Drop or paste the items into your new playlist.
3) Select/hightlight the playlist
4) Go to File--->Export Playlist.
5) Choose a file location and title.
6) Select "HTML" for 'Includes field data'.
7) Do not select anything under 'Filennames only'.
8 ) Select "All Fields" under 'Included fields".
9) 'Output range' should be set to "All"
10) Press OK
You can open the resulting HTML file in Excel, and it will be formatted as a spreadsheet. Depending on the size of your library, the file size could be massive. If you re-save it as an Excel file (XLS, or better XLSX if you have Excel 2007), the size will be reduced significantly.
You'll need to delete the top row or so, which simply reads "Media Center Export" or something like that, as well as the row just below the end of your table (which also reads 'Media Center'). You'll also need to select all cells and "unmerge" them (your data won't be affected).
I do the above frequently. It's great! I can pivot table the data, look at it in different ways.
The only downside (I believe---can someone confirm??) is that you see only the tags that MC is set up to see (all default fields and all user fields). For example, I have a user-field called "Totaldiscs". In reality, with FLAC files, this is stored within the file as "Totaldiscs". However, in MP3 files, it's stored as "JRCOMMENT:Totaldiscs", or something like that. MC knows this, and for sake of convenience, it just displays the one "smart" field, which is what the export shows. In other words, the export shows all of the library tags, but not the tags within the files themselves. This is either a very good thing, or not so helpful, depending on your point of view.