I bought one on Cyber Monday for $119.

The first thing I noticed is it has a nice wide bezel on it perfect for little hands to hold and not turn pages. The display is warmer than the HDX 7's blue tint. Software wise everything looks just like a HDX just no Mayday button!
Yay!! I found Mayday to be something I wouldn't ever use. The two times I did when I was trying out the HDX the people were clueless and I had to help them.
It has Goodreads integration

and all the extra Text to Speech voices can be downloaded. It has X-Ray Music and Movies. The buttons are the same too. Other than form factor, weight and hardware there isn't any difference from the HDX.
The text rendering between the HD2 and the HDX 7 is almost the same. I was shocked!
The sound on the HD is much better than the HDX 7. It's louder no need for a case to amplify the sound. I listened to it play music all day and had to turn volume down several times. On the HDX I had it near max to hear anything when it was out its case and this was on all 3 that I owned.
I saved the best for last
No blue border! It uses normal LEDs. There is some light bleed on one side that's
barely noticeable when on a black background but on lighter backgrounds not noticeable at all. My first Kindle Fire has a similar edge to it. The N7.2 I own has no light bleed whatsoever.
I bought the Poetic Slimline case ($5) for it but it fits my Official Kindle zip Sleeve despite reviews saying it doesn't You just have to lightly tug the zipper away from the top corner and it zips right up. I like having options.
All in all I am very happy with my Kindle Fire HD2

I really like it much better than the HDX 7s I owned.