For me it was Microsoft’s COM and how it’s all powered by interfaces. It took a long time before it clicked for me. Fortunately, it didn’t really matter because

For me it was Microsoft’s COM and how it’s all powered by interfaces. It took a long time before it clicked for me. Fortunately, it didn’t really matter because 20+ years later, it’s still relevant. I’ve had plenty of time to work it out. The other Windows-related idea that took an embarrassingly long time to work out is device / printer / memory contexts.