1. What technology is behind this? Ruby?
2. Mobile is VERY important. So important, in fact, that if I were you, I'd make a mobile version of the website right now to work across Android/iPhone/webOS/Windows/etc and start working on an actual native app asap.
3. Syncing. I'd love to be able to Sync this with iCal and Google Calendar. Of course, this is a huge thing; syncing is a lot more difficult than most people realize.
4. Mixing To-Dos in there with the Calendar would be killer. Especially if you let me bring over my To-Do list from popular To-Do list websites and then make your own version.
5. I love the fact that it's so simple and nice. Please keep it that way.