Items that come to mind for those making the transition:
Skills
Managing Expectations: As a leader, chances are you'll still have someone that you report to, whether that's a more Sr. Manager/Director, an executive like a VP/CTO/CEO, or your investors. It's important to manage expectations upwards to keep the necessary parties informed and aware. Nobody wants to be surprised by skeletons in the closet or a project running late.
Setting Expectations: As a leader, you will no longer be measured by your individual contributions, rather the cumulative output of your team(s). It's important you provide your direct reports with the proper instruction and guidance for them to deliver on the objectives of the business or project.
Accountability: A team is only as strong as its weakest link. As a leader, you'll need to hold your team members accountable to the expectations you set. If you don't hold your team members accountable, then the team can easily devolve into anarchy costing your business time & money.
Detachment: As a leader, you'll sometimes have to deal with uncomfortable tasks, like terminating a employee. It's important to understand that some level of personal detachment may be required to execute these tasks as a working relationship may begin to feel like a friendship.
Coaching: Leadership isn't just about getting employees to do the work, you want to see them grow as an individual in parallel to the growth of the business. Ideally, you find ways to intertwine the two for growth paths for mutual benefit.
Feedback: Since none of us are mind readers, it's important to engage in feedback cycles with your employees to shephard process and behavior changes.
Ownership: The buck stops with you. Understand the scope of what you're taking responsibility for so you aren't blindsided.
Experiences
Letting Go: As a leader, you'll need to let go of some/all things you did as an individual contributor to give capacity towards leadership tasks & responsibilities. In my current role (Sr. Management), I no longer contribute code towards our products. I still get some opportunities to write code annually, but those are small, tactical projects that benefit my team members or the company internally.
Hiring: As a leader, you'll have opportunities to pick your team, knowing how to spot the right talent for your style of leadership is important. There's more to a candidate than simply whether/not they can do the work, and it can make the difference between a valuable team member and one that drags the whole team down.
Underperforming employees: Whether it's a bad hire, or an employee who's engagement has slipped, it's important to go through the experience of dealing with an under performer. Knowing how to address those situations, and executing quickly is critical to minimize the impression the underperformer will leave on the team and your reputation.
Setbacks: Sometimes as leader, you're gonna be hit with bad news outside of your control. How you adapt to the situation will harden you as a leader.
Items that come to mind for those making the transition:
Skills
Experiences
Incredibly detailed and helpful answer, love it!