I had to buy a nook for some ebook testing, and the thing that bugs me the most is that you can't turn the page forward, using the touch screen, with your left hand. With the kindle, I can read with just my left hand - you just have to reach past a very narrow strip of the screen which turns the page backwards, to get to the area (most of the screen) which will turn the page forward. But with the nook, you have to touch the right third of the screen to page forward - impossible to do with your left thumb (while holding the nook with your left hand).
Yes, it has page turn buttons, but you have to press them really hard, and since the nook has a kind of rounded back, the nook would rock quite a bit whenever I tried using the buttons.
And finally - what the others said. The kindle paperwhite has higher resolution, whiter white, blacker black, a better light, and costs less.