You won't get me lumping out cash until it works like paper books:
* I can buy content from whoever I want anywhere I want
* The content becomes my property in a tangible way
* I can give, loan, swap content with other people
* I can give it to Oxfam who can sell it half price to students
* Oh and content has to be a lot cheaper than a paperback, to reflect the supply sides lower costs.
* I can generate my own content and give it away to friends
On the way, a standard memory card slot, a standard USB connector (also used for charging), and a week-and-a-bit battery life so I can take it on holiday would be essential too.
All that said, I quite like the idea.