Lighting is easily the biggest hurdle I encountered trying to build games as a solo dev. It's not particularly hard to understand the concepts, but it's quite unintuitive and labour intensive to get good looking lighting. It was also very prone to engine-specific bugs or tweaks in obscure settings, very frustrating.

I guess like many industries as these new technologies come in there will be old-hands who lament how easy it is for newcommers, but if I could do away with the whole lot and just look at lighting as if I were dressing a movie set, and materials acted exactly as you'd expect, it would be so enabling.