Back in December, I got cooped up in a winter cabin, and so had way too much spare time to test my NT vs. a Kindle Fire that someone else there had.
We swapped devices a lot- the KF owner was way more impressed with my rooted NT than I was with the KF. Of course, it was rooted vs. stock but many of the noticeable NT advantages weren't strictly related to that.
Speed: The NT is snappier overall. It's definitely faster playing 3D games.
Screen: To my eyes at least the NT is slightly better.
Build quality: The NT is sturdier than the KF, and drop tests have proven it. When you hold it to read for long periods of time, the outer bezel is more comfortable in your hand.
Storage: No contest. The most perfect real demo occurred when a friend shared the movies he had on his phone's micro SD card. No problem with the NT. KF was left out in the cold. And of course, no wifi, so no netflix or other cloud services so internal storage was king. I had already brought tons of movies, music and books on my own 32GB card. The only way to share with the KF owner was to trade devices. (The owner had a laptop and could have transfered via that except for: no microSD slot or converter.)
Battery life: NT wins here too. The KF spent a lot more time off in another room at one of the few available outlets being recharged than my NT did.
Suffice it to I was not all that impressed with the KF. It's not a bad device, but in a situation where you're actually using either a lot to stave off boredom, and therefore want a lot of content choices, consistently smooth performance, and battery life, the NT just stomps all over the KF. I don't regret getting it vs. the KF for a moment.