I think people need to remember this is a e-book reader forum. All devices are represented here, not just the Kindle.
This question really should be:
What do you like best about your
Ebook reader device?
Now to answer your question.........
I love the knowlege I'm carrrying every book I 've bought/downloaded with me, whereever I am.
I love the built in dictionary.
I love being able to change the size of the words with just a touch of the button.
I love to turn the reader off, then having it open to the same page I left off when I reopen it.
I love being able to read a really heavy book, (ie The Sherlock Holmes Omnibus) with a device the size of a paperback.
I love being able to read in bed, without cramping or trying to get comfortable.
I love being able to read library books, legally, on it, knowing they will simply 'turn off' when the lending period is over.
I love the looks of my reader.......which happens to be a Kindle!