The biggest thing that I'd say about shifting from a developer to a manager is that you better like dealing with people, if you don't you will be burned out very quickly and you'll be a sub-par manager.
You're going to be responsible for helping your subordinates get their stuff done and progress their careers and you better be the kind of person who can put others first or you're going to begin losing people, which only makes your life more difficult.
Firing people really does suck, but laying people off is gut wrenching. If it's a surprise to them that they're getting fired for not cutting it, that's a sign you weren't being proactive enough with the feedback (or that they're completely clueless which does happen). For me, when someone isn't cutting it and they're teetering on the edge I have a very blunt conversation letting them know they're on their last chance and exactly what the problems are. Usually by that point it's the 3rd or 4th time we're having the same problem and they're all but out the door, but I've had people miraculously turn it around. If you have any empathy this will be the worst part of your job.
Resourcing if that's something your company does will take up the majority of your time if the company is busy- everyone fights for the 'rockstars' and you'll need to figure out how to match your people with projects that suit their strengths AND interests.
You've got to be putting in the 1:1 time to know where your people are interested in growing their skills out and identifying what their strengths are. You've got to be able to figure out how to put people on projects that might stretch their comfort zone and also be able to support them so that they don't fail and keep an interest.