First of all, regarding the question: "What is my top responsibility as a team lead? What should be the No. 1 in my TODO list?". The simple answer is:
Keep asking myself how my team has improved in the past interval: how my team has improved in becoming a team, how my team has improved in doing things, how my team has improved in becoming a super team, how each member of my team has improved himself/herself, how did I improve myself for my team, how did I improve myself for myself...
If I am not at least thinking about that, I am just a programmer with special attributions!
Secondly, I realized that nothing can improve out there in the comfort zone. If I'm leaving you in your comfort zone, and I stay in mine, we might be more comfortable with each other, but we won't in any way progress in anything.
So If I don't have the guts to challenge somebody or myself out of his/her/mine comfort zone, I am just a programmer with special attributions!
Thirdly, I realized that I can easily become an impediment for my team. Yes mister Scrum master, you can easily become an impediment for your team! If somebody comes to you complaining about the WiFi in the meeting room and you don't ask him "what will you do about it?" but just forward the question, you're on the right track in becoming impediment.
Fourthly, I realized that broken window theory doesn't apply just to failed unit tests but to any practice you would like to have in your team. Skip a code review once, and code review will become optional, skip a retrospective, and all the retrospectives will become optional or nice to have, just break the window and everybody will know it's save whatever they want.
So if you have an hour, enjoy the movie, it contains also other cool stuff: Code Leaders and Beautiful Teams
The guy's site is pretty helpful too: 5whys.com