The Nook and the Fire are "Android" devices not Android™ devices. While related and important they are not the Android ecosystem most people talk about.
I would argue that this distinction is not that important from developers' point of view, as the same application can be made available on all "flavors" of Android.
It is important from a developers point of view because those devices don't get tested by Google for API/feature conformance and so potentially are more likely to have applications not run on them correctly.
Given the wide variety of applications shared amongst the three markets (plus the successful installation of Google services including the Play Store on both), I think there's zero evidence that, from a developer's perspective, the Nook Color/Tablet and Kindle Fire are materially different from any Google-blessed Gingerbread-based tablet with the same specs, except that the Google-specific libraries and services are not installed.
I can understand compatibility worries about cheap, no-name non-Google-certified tablets, but that doesn't describe the Kindle Fire and Nook Color / Nook Tablet. The cheap, no-name tablets (and other devices) aren't a big part of the market outside the developing world. And the developing world is a whole different ball game for all sorts of reasons, not just Android device compatibility (e.g. the most significant competing platforms are Symbian and, arguably, SMS).