I know I sound like a broken record here because I suggest it all the time, but one of the biggest eye openers for me was when I realized the correlation between the makeup technique "contouring and highlighting" and lighting for portraits. The makeup technique is used to sculpt and highlight certain facial features to make someone look their best, and applying the idea to the way I use light does the same thing, simultaneously reducing the amount of retouching needed in post, which to me is a huge deal because I hate spending too much time editing one photo.
The other major eye opener for me was discovering just how amazing and versatile the dodge and burn tool in photoshop is. If I could only use one editing function in photoshop and nothing else, it would that, and I would still be very happy about it. Hell, I'd still subscribe to CC if all it came with was a dodge and burn tool because of how incredible of a skin retouching and sculpting tool it is. I feel that every portrait photographer should learn how to effectively dodge and burn.