How hard would it be to use the mic input to estimate progress through a piece and turn the pages? Of course just using timing to scroll at the right speed would be ok, but having it listen would be much cooler.
It's a fun app for the common folk, but useless to experienced players.
The following part hard is a bit harder, but not impossible - compare what you have recognized with what you was expecting. If it's not what you expected, look around the last known position and see if you can find the pattern you recognized (while taking repeat signs etc. into consideration).
The highlighting of the keys is also an amazing feature. I've posted YouTube videos of me playing the piano with the camera angled unintentionally over my shoulder.. and actually used the video to see what keys I was playing and copied what I was doing.
There should definitely be a "no sheetmusic" mode in this app for people who previously copied people's finger position on youtube... for people who have no desire to learn how to read sheet music.
Good luck.
• Typesetting: I don't know what score format they're using, but this can be a big one. There's a big difference between a professionally engraved score and just having the right notes there.
• Annotations: From the conductor, self, whoever.
• Overview: Professionals scan a score looking for key parts. This seems virtually impossible in iPhone format, but would be workable on the iPad.
• Size: Even the iPad is small compared to a performance sheet music page.
I suspect the three big market segments are hobbyists, children and church musicians. There are relatively few professional performers.