You might think it's the e-ink screen. It's not.
You might think it's the ability to carry around hundreds of books in one device. It's not.
You might think it's the lower cost of brand new books. It's not.
So what is it? The best thing about the Kindle is the delivery mechanism from Amazon. It's called WhisperNet, and it means that I can buy and download books to my Kindle without ever using a computer.
Not sometimes, not rarely, never. And while I have an SD card in mine, I never need to take it out and stick it in my computer to transfer content either.
The way it works is that I just turn on my wireless chip in the Kindle. That's done with a small switch on the back. I tend to leave mine off because I rarely buy books and it does affect the battery life. With it off, I can leave my Kindle on for a week or more between charges. I read 100-200 pages a week, and most of the time the Kindle is in screen saver mode, but I have rarely run even below halfway on the battery.
When I want a book, I can easily select "The Kindle Store" from the menu and it connects to Amazon.com and allows me to browse, search for, and purchase books. I have a credit card linked to my account, and can, and have, change that if I wish. I also get gift certificates that I apply to my account, and they are used for purchases from the Kindle before my credit card is charged.
Once I find a book I want, I click "buy" and it downloads to my Kindle in literally a minute or two. It's often downloaded by the time I go back to the home screen.
But I rarely browse the store and buy a book. Typically what I do is get a "sample" from Amazon, which is about 30 or so pages of a book. I just grabbed The Autobiography of Theodore Roosevelt recently and it was 60 pages! That's a long time to decide if it's worth getting the book. I had the font size set at "2", for those of you with Kindles.
For the rest of you, not sure how this compares to the real book. I'd have to purchase a real copy of something on my Kindle and compare. For the Teddy Roosevelt book that was all of chapter 1 and partway into chapter 2, which seems about what I find for many books. I get 2-3 chapters unless chapters are really short, like a James Patterson novel.
This "purchase on the go" is amazing for me. I can actually buy things when I'm out and about, and don't need to store up books on the Kindle. I typically have a few samples on there I haven't gotten to, or which looked interesting, but I can buy new books almost anytime, so I grab them as I run out of stuff to read. I even bought two books when I was in an airplane in Houston, sitting on the tarmac and I was almost finished with a book, so I grabbed two more before we took off!
There's one other thing that amazes me as well. I can buy Kindle books from Amazon.com, on my computer, and the next time I turn on the Kindle wireless connection, it will download those purchases.
Even more amazing, I can send samples of books to my Kindle from the computer. Today I was looking at something and got interested in Theodore Roosevelt, so I browsed over to Amazon.com, searched, and then picked 3 or 4 from the Kindle store and sent samples to the Kindle. When I went upstairs, I turned on the wireless and walked away. Later I was working a bit more and wanted to get some science fiction as well, so I sent 4 or 5 more samples of new authors over to the Kindle and by the time I'd walked upstairs to check on my Kindle, the samples were downloaded, and I turned off the wireless.
Amazon has a fairly captive audience, and it's likely that the majority of my book purchases from now on will be from Amazon. Since I read about 50 books a year, that's a decent amount of money they'll be getting from me.
However I have to say that not carrying around lots of books, not needing to find time to get to the bookstore, or having something out of stock is a fantastic convenience for me.