I've had the original Kindle Fire and a friend of mine has the Nook tablet (previous model). Both are very much locked down.
I now have the Nexus 7 and it's the best 7" Android tablet in my book. Very zippy, very sexy, clean design, great screen, great price, and you can install all reading and library borrowing apps on it: Kindle, Nook, Kobo, Overdrive, and non-US book sellers' apps if you are so inclined.
The Nook and KF simply don't let you do that, unless you are willing to tinker with them - which you shouldn't have to.