The fact that the next version, which comes 2 full years after the launch of Android 4.0, is not going to be Android 5.0, but actually Android 4.4 took us all by surprise, because we have to ask what is going on there that it's taking Android 5.0 so long to arrive? Is it going to have a major UI overhaul? Will it switch to 64-bit, and it's taking this long to refit the Dalvik VM for it?
We don't really know, and that's a little worrying, because we're hoping it's not because they had major setbacks for different reasons. But until it's released, we can only hope it's because Google intends some major changes for it, such as a more card-oriented UI. That's what this concept video is showing, too, a much more card-oriented UI:
As you can see, the video is showing cards throughout Android, and not just cards for every piece of information, but also a "card-stack" multi-tasking UI, which makes sense for where Android is heading, and it would also make it a little more like webOS (Matias' baby) used to be.
Google has already hinted this is the direction they are heading, with the new Youtube and Google+ apps, which have not only a card-based UI, but also seem much faster performance-wise, even on lower-end devices. If they are showing those new changes this early, we can begin assuming that these changes could land in Android 4.4, and we won't have to wait for Android 5.0 to see them.
If Android adopts the latest Linux kernels, we should also see a lot more performance throughout the OS, and also in all the new card-based apps. There have been some rumors earlier that the next version of Android will be optimized to work very well even on very low-end hardware, so it's likely that will happen.
Android 4.3 and Android 4.4 didn't seem like huge improvements, but it seems that Android 4.4 will be at least as big as Android 4.1, if not even as big as Android 4.0, unless they really want to keep most of the UI changes for Android 5.0. Android 4.4 is rumored to come out after October 14, when the new Nexus 5 is rumored to be announced, too.