Inheritance; many Qt provided objects can (and should) be extended by you, to provide more functionality while still being Widget objects that can be displayed as expected and interact with other Qt objects. Coupled with this, how to feed an object of derived type to something expecting an object of base type, without slicing it or otherwise shooting yourself in the foot. Qt tends to pass QObjects around with pointers, which does make it harder to get this wrong.

Objects; they're all objects. Understand objects.

RAII/Constructors and destructors; managing object and resource lifetimes, and suitably ensuring that visual widgets and other such Qt objects are created and disposed of properly.

Threads; Qt Widgets (and your own objects) spark actions in Qt objects through signals and slots (fancy Qt meta-programming that takes care of a lot of boilerplate for you), that have rules about working across threads. Understanding threads is needed to understand these restrictions, and the GUI rule of "stay off the GUI thread" applies always, if you don't want to lock up the GUI. This means you'll need to create other threads to do the heavy lifting. You don't have to use standard C++ threads, but you'll need to understand threading.

Don't use QList unless you really have a good reason, even if you find old documentation saying you should.