Many people, especially those with strong visual and design skills (a group I am sadly not a member of), have opinions on how their books ought to look, and the idea that on the Kindle, they can’t specify exactly what font to use is a bit disconcerting.
The reason you can’t, is of course, because the user can specify what font and size to read with, so it’s best to leave the user their default, rather than interfere with it. This was really brought home to me when reading the comment of a former elementary school teacher of mine:
If your eyes are not what they used to be, the Kindle DX is for you! (I can make the print any size that I want and the screen has been improved so I am able to read longer without strain).
For people whose eyesight isn’t what it used to be, and yet remain avid readers, the Kindle is giving them a new lease on being able to read. As someone who loves reading myself, I can only imagine how wonderful it must be to go from gradually finding reading more and more tiring, to being able to read easily and comfortably again.