Let's focus on e-book readers for a moment. People are looking for an ebook reader that can do more than just be used as an electronic book library. They would also like it to play mp3s (audiobooks, music) or maybe even videos (movies, streaming video episodes of TV series, etc). I can see the appeal for that. But apart from mp3 and video storage, should an e-book reader be able to check email and browse the web? I am not so sure, and here's why:
Reading a book is probably the best single example of a single-tasking, focussed action a person can do. You read a book, you get into the story, you lose track of time and nothing else matters. We all hate it when you are right in the middle of an exciting book and the phone or doorbell rings.. What the hell, who disturbs my moment of alternate reality!?
Imagine an e-book reader that checks for email and goes PING if there is an email. Or even worse, shows a notification right across the screen. Yes, you can probably disable this, but the nagging "maybe there is new email" may stay with you throughout the book, ruining the experience, pulling you away from that wonderful virtual world only a book can create. And we know how distracting Twitter can be. Say your e-book reader supports browsing. I bet you want to check on your friends' tweets once in a while. Stop reading, check the web or twitter. Another disturbance.
So, maybe e-book readers should remain just that. For reading electronic books. Undisturbed by other features. Okay, apart from an empty battery once in a while...