I was shocked at how obvious my mistakes as an engineer became after my perspective changed. Taking on manager responsibilities for the first time revealed mistakes I had made as an engineer. Here are the top three things I could only see after becoming a manager.
Soft skills are incredibly important, but so is understanding that not everyone handles "social events" well - as a manager I recognise that members of my team have varying levels of ability to endure social interactions, and use that knowledge to help them on their journey, without overwhelming them
One thing I learnt is that no matter who the engineer is they love to have someone to listen to about their work, so that is 1 to 1 socialization I do as a junior engineer.
One mistake I made that I don't make now that I became a business owner is that you always need to ask yourself -- does what I'm doing generate sales and profits or not. Otherwise, why am I doing it in the first place?
I've heard from others that it's important to have soft skills (communicating, connecting with others, etc) with technical skills. Just having technical skills really isn't enough to become great.