Exposure was a big of a learning curve at first - realizing that Aperture, Shutter Speed and ISO all work cohesively and changing one affects the others. Understanding Auto focus too... if I got a sharp photo of something moving, I was excited because I had no idea what I did to achieve it.
But for me, it was more on the creative side. I realized I could take photos pretty easily and it took me years to come to grips with the fact that very little matters (at least for me) on the technical and what I put in front of the camera was more important. I had it the other way around at first 🙂