With the Nook you can highlight stuff in a book, attach notes, and recall the highlights and notes anytime you want. It also can play mp3 files, and has a small set of speakers built into it, along with a headphone jack. You can listen to mp3's while reading the ebook as well. You can load .pdf files through the included adobe app, but you can't highlight the pdf document, at least not yet. I'm sure there will be a hack for that soon. With the Nook, you can swap books from your laptop/desktop back and forth, and if a friend has a Nook you can beam the title to their unit for them to use for 2 weeks, then it reverts back to your device. Money magazine just did a write up on the Sony,Kindle, and Nook and the Nook was the clear winner.
Additionally, you can read magazines and newspapers on the Nook, as well as Barnes Noble exclusive content while you are at the store. From what I've heard, you can also read a title in the store for an hour before deciding the buy it. Not to mention the million + titles available for it.