First, let's talk about the K4 and the problems it has had with apps. Some time back I noticed that not all apps work for the K4. At the time I had thought it was a hardware problem (RAM limitations or lack of a keyboard) was the issue. Nope.
It turns out that the first reason some apps work on the K3 but not on the K4 is that the developer didn't request that the app be tested for compatibility. The same app will run on the K3, KDX, and the K4 just so long as the developers tells Amazon to check. Edit: And so long as the keyboard and other hardware differences are planned for, of course.
If you don't believe me, take a look at the NotePad app from 7Dragons. When the K4 shipped this app didn't work on it, but it does now. Of course, this app isn't all that usable on the K4 (no keyboard), but at least now Amazon will let you use it.
And then there's the K5. Apps that have been developed for the K5 are based on an entirely different code base. All the other Kindles can use apps made with KDK 1.0 (or 1.3). On the other hand, the K5 can only run apps that use KDK 2.0. And that's probably because of the touchscreen and differences in the software.
KDK 2.0 was only released to developers last week, and that's why at the moment the K5 has so few compatible apps. Most of these apps are from Amazon's own developer team and from a select group of developers who had early access. I've found 3 developers (not including Amazon) who already have a bunch of apps for the K5. They are:
I'm sure there are more developers, but these are the 3 that clearly had early access to the development kit. Or they just happen to work really fast and got Amazon to approve the apps really quick.
And as for the apps that work on both the K5 and the rest of the Kindles, that's because the developers built the app twice.
In any case, it's quite clear now that Amazon has split the Kindle platform in two. On one side is the K5, and on the other side are the rest of the Kindles. Doesn't it seem rather odd for Amazon to put all this work into the one Kindle model? I think it's odd, but I also think that the K5 is just the first touchscreen Kindle. There will be more.
I'm expecting to see the next Kindle DX show up with an IR touchscreen - that is, assuming that Amazon makes one. (I'm not sure they will.)
Let's consider for a second a KDX Touch. It could come out next spring. Assuming Amazon matches the current KDX price point, we'll have a 9.7" E-ink tablet selling for $380. It will have the same pinch zoom and scrolling features found on the K5 and by the time the KDX Touch is released we will likely also have drawing and sketchpad apps.
Of course, before you get too excited about the mythical KDX Touch, do remember that by the time it comes out we will also have 10" tablets selling for $400 (or even less). Those tablets have much better specs than the KDX Touch, which makes it hard to justify going for the limited hardware. Also, the small market for this large screen ereader might preclude Amazon ever releasing it.
But I hope they do. It would be neat to see.