Well, the Kindle has a very clean UI. I absolutely despised the home screen for the Nook the one time I borrowed it from a friend for comparison -- IIRC it divides the home screen into recommendations, and separate links to your last read book and to your main library view. I gave up right there. The Kindle doesn't mess around with that -- the home screen lists your books directly. And it allows you to turn off the new recommendations "feature" on the cover view -- I respect that a lot. My KT seems very responsive, and Amazon has a reputation for rock-solid firmware stability, so that is probably to be expected.
Navigation: one tap on top to open the menu popup, another tap to select the Go TO menu, which lists all chapters (with nested ToC support) as well as a "Beginning" link and an option to specify a location number (not very helpful) or a page number (only if the book supports this via the sidecar file. Most books bought from Amazon will have one.)
Book sorting in:
- library view -- your options are Collections/Title/Author/Recent. inside collections, you can sort by Recent/Title/Author.
- search view -- Number of Hits/Recent/Title/Author.
Sideloading via browser download, Send to Kindle, or USB sideloading. USBNetwork hack can add books through SSH over WiFi. limited format support.
Highlight any word and in addition to getting dictionary definition, it will offer you the Wikipedia page as well. This is also allowed over 3G (Amazon WhisperNet) in supported devices.
Overdrive is pretty built in -- you can access their site via the browser and connect a book directly to your Amazon account, it is by far the most seamless OverDrive experience -- if the book is available, but many of their books gained Kindle support recently.
BUT there are very few customizable options for your reading experience. Kindles are designed for clarity and the least number of confusing options. I guess they should "just work".