Depending on the book, there might not actually be enough room to discuss things with the author. Some classic anthology books have font-sizes only suitable for pixies and ants.
There are ways to markup a book without needing a journal, or irking the next person you resell the book to though. Here is a link as an example:
http://www.coolhunting.com/design/transparent-pos.php
I don't think the ability to lend a book only once for 14 or even 21 days is good enough. When you give someone an old book, it stays with the other person unless they give it back. I'd be willing to pay a small transfer fee for giving my ebook copy to someone else just because someone has to build and maintain a system to make such a thing possible.
Ironically, Amazon and their ability to retroactively delete ebooks without your consent seems to me like the company with the infrastructure most suited to make such a thing a reality. Of course, ebook sellers could simply rely on the honor system, but that would need a majority of customers to be completely honest most of the time.
There's just something to be said about giving someone a book out of your own collection instead of buying them a copy or sending them a gift card.