Q.How would people share notes? A) Could be live sharing in class (etherpad for ex: http://piratepad.net/sj8l1FIUIK ), then contents of pad get transferred to a wiki and people are free to edit.
Q2.Why would people put in the effort to type-in their notes? A2) Therein lies the trouble...
If you can think of a business model, or an incentives structure that would make this a open collaborative project, do pm me. Note: It would be against my values to put ads all over site to monetize.
To write the latex file, they worked together (sharing the same file, same idea as google docs).
And at the end, they put the effort only because they thought it was nice to have all the notes in pdf, commitment with the class, and because it was a good idea to share it with the other students. I still have all my mathematic classes pdf's.
A2 : Incentive model may always not necessarily involve monetary benefit.
PS : Check mail.