I love the checkbox-function, but like to have more variations:
1) check-box advanced; not only 'checked-unchecked' but also a third option for "tasks on hold"
2) trafficlight; to monitor the status of my projects (green=on schedule / orange= slightly behind / red=big problem!)
3) 1-5-star-box like the one you see on sites like Amazon to rate items. Nice to give my own scores in the notes with my book/dvd/cd-collections.
4) Statusbar 1-100% to represent for example project progress
Maybe other users have other great variations in mind?