Excellent idea!
Instead of choosing either dropdown (or slider) vs. text-entry field, I suggest that you allow both. If you use the dropdown (or slider), the resulting data would be shoved into the text field, or if you enter something in the text field it will override what the program picks up from the dropdown or slider.
Concerning the date field, default it to the current date and time, and allow override (Ole has a good idea for using a popup calendar).
Add GPS coordinates (or some other location information). I'm not familiar with the EXIF specification, so whatever it calls for in terms of location.
When you get an alpha version ready, be sure to post it here and I'm sure we'll kill all the bugs for you! Ha!
-Joe-