First and foremost was the build quality of the table. Compared to my older Nook Tablet and even my Nexus 7, the HD+ is a very well-built table that exudes quality. It never flexed or creaked when handled, especially when it was put into and then taken out of it's book-style cover. The external frame looks to be made of metal (either aluminum or magnesium), with an excellent glass display on the front and non-skid back. From a material and construction viewpoint it stands head and shoulders above everything, including the Apple iPad.
The version of Android used on the HD+ was a customized version of Android 4 (Ice Cream Sandwich) with Barnes & Noble's own custom UI overlaid. The UI coupled with the high resolution of the display was gorgeous. The software, at least at the top levels, was responsive and fluid, almost as fluid as Jelly Bean (Android 4.2.1).
Barnes and Noble continues to use a custom version of Android, in this case 4.0. That means no access to Google Play, only Barnes and Noble's very limited app store. Because this was running Android 4, the built-in versions of Mail and Browser were limited and still contained bugs that were fixed in later versions of Android; Mail has grown considerably and the built-in Browser was replaced with Google Chrome. Even apps that were available in the Barnes & Noble store as well as Google Play, such as Flipboard, were slow and buggy while running on the HD+. The only positive feature was reading books and magazines; there it was at least decent. But this is a tablet, not just a reader. Every time I encountered yet another bug that I knew was fixed in a later release, I grew more angry and less satisfied with the HD+, until I finally returned it.
The HD+ is physically a great tablet marred by the political/business decision on the part of Barnes & Noble to hobble it with old and limited software and a poor app market. There is no excuse in 2013 and beyond to not allow access to Google Play, if for no other reason than to get proper bug fixes and better applications like Google Chrome that will run on Android 4 and later. Returning the HD+ was, believe it or not, the hardest return I've ever made.
Oh. One more observation. I installed Flipboard on my older Nook Tablet, and it works better on it (running a customized version of Android 2.3).