My biggest complaints about ExtJs is that it has an extremely difficult learning curve and, while the docs have gotten better, they still don't cut it. For example, the venerable ComboBox is rife with pitfalls. I've since forgotten the details, but the various config options interact in strange ways.

My biggest complaint is that the template holes in components aren't html escaped by default. You end up having to override the default templates just to add "htmlEncode" liberally. For example, the venerable ComboBox should just escape the items by default instead of allowing html in the combobox menu items by deafult.