I am the music librarian for a big band. We have nearly 300 arrangements in the 'current issued pad'. Since we often play outdoor gigs, these tend to be kept in 4 ring binders in plastic wallets (thats 2 x 2 1/2" A4 binders weighing in at about 5 kg)... am I glad that I play Saxophone & not Keyboard which has at least double the pages!!!

All the music happens to sit on my laptop as PDF files - but I am reluctant to put a £2k laptop sideways on a music stand 4 foot off the ground in a typical British summer. I can read / play the music off the laptop easily enough using Acrobat Reader - but page turning is awkward. Also, switching the laptop from landscape to portrait mode is a pain (thanks Dell)- so that is not suitable for gigs.

Several in the band have switched to iPad using the Forescore or GigBook apps. These both seem to have fixed the page turning problem (you just flick the page over like turning a book page). One even has a foot pedal connected so can turn pages whilst keeping fingers on the instrument. iPad cases also have the advantage of being magnetic - so they stick to the stand. Cheap apps - pricy iPad - but so much lighter.

My only issue with iPad is the screen size - too small and the rumours are that the next generation may go smaller to 'compete' with kindle.....

I would say that the functions you need would be:

Start with PDF files (it would be nice if you could shuffle pages & rotate if necessary too)
Easy page turning
Easy way to skip back to a DS mark - or on to a coda (could be several pages)
Index system
Setlist capability (select a list from the index to play in a defined order)
Ability to add mark-ups (highlight the places where you forget the accidentals, etc)

(I have not yet found one product that supports all of that)