Marco Arment writes up a nice comparative review of the Nook versus the Kindle 3, complete with screenshots and measurements.
As a recent, happy purchaser of a Kindle 3, I'm pleased to see the features I care most about are either exactly the same or missing in the Nook. The killer missing feature for me though boils down to this:
While this will matter to very few Nook owners, I can’t use Instapaper or any other online service to send web content to the Nook: there’s no document-delivery mechanism like Amazon’s email gateway, and no web browser to download files directly. The only way to get non-B&N content onto the Nook, as far as I can tell, is to transfer it via USB or a micro-SD card.
In my mind, the number one feature is the screen, but a very close second is the ability to email and transfer a variety of documents to the device for easy reading later. It's allowed me to take all of the online reading I do offline, which is exactly what I wanted.
Oh, and speaking of Kindles and such, you should visit the subscribe and check out the automatically-generated MOBI version I've setup. I don't have a mechanism that will deliver it wirelessly to your Kindle yet, but you can download the blog in its entirety and transfer it yourself.