I took the 4.1 RC3 and tested it with my apps. Most of my apps change the data store page size to match the physical size of the grid shown. This worked in 4.0, but 4.1 throws an exception:

Uncaught Ext.Error: pageSize cannot be dynamically altered
How should I alter the page size in 4.1? I want the grid to show just as many rows as fits in the browser window...

By the way, there's a typo in data/Store.js:

Ext.error.raise("pageSize cannot be dynamically altered");
I suppose that should read:

Ext.Error.raise("pageSize cannot be dynamically altered");