Although Sencha does a good job with introducing different interesting things, I am completely dissatisfied by the support of existing users.
In their blog post they say:
With its API frozen we have no hesitation in recommending that you start building your apps using B3 if you’re keen to get a head start on your app.
As far as I understand, this means that they are not going to change the API - remove or add methods and properties and migration to the versions after Beta 3 should be as easy and replacing the framework files.
Here's what I discovered.
They made us specify view, stores and models in app.js. This is an awful idea. This is not only my opinion. Jay Garcia started a topic about this recently.
I was hoping they will not introduce this. But they did and removed the back-compatibility.
I hate them for this decision.
My workaround is to specify views in the requires section of controller otherwise I will end up with 150 views in my main file.

I don't think, this change falls into "frozen API" definition...

Data package is behaving strangely, grouping in the stores is broken, rendering of the docked items in the viewport is strange, getEl() method that has been in ExtJS and SenchaTouch for ages is gone, main promise of the Sencha Touch about targeting different screen dimensions is nothing but words...
Instead of focusing on those really important issues and poor performance of the framework, especially on Android (Galaxy S, 1GHz) they focus on some things that are nice to play with.
Is it serious approach to the best Javascript framework of the world???