It's important for the leader to delegate the daily (team management) responsibilities. For example:
-hosting / organizing events
-recruiting / approving new members
-helping settle any disagreements that might occur
It should absolutely not be 1 person making every decision or organizing every little thing for the team IMO. I believe the devs made the base team roster layout as a pyramid (1 boss, 3 executives, 5 commanders, etc) intentionally, and embracing that pyramid structure seems to work well. I think the higher ups (executives, commanders, even officerS) in the teams should be allowed to take on more and more responsibility as they climb up the structure & as a leader you should give away most, if not all, of the power / responsibility to the higher ups in the team. You should be there as the final say in an argument or to handle more serious situations. The team should function mostly autonomous in that aspect so you (as the leader) can focus on the best strategies for keeping it alive and healthy long term. Not to mention, if the leader tries to handle every single situation / event / etc. on their own they are going to burn themselves out quickly.
I very much agree with what @OrangeManiac said about a team having a clear purpose. I don't really have much more to say about that topic other than that the purpose/focus of your team can change over time, and that's OK. For example, my team is 7 years old. It started as mostly shiny traders (when shiny trading was more popular / common than shiny hunting), and over the years we have reshaped it into a team of shiny hunters instead. Your team's focus can be whatever you and your team members want it to be, as long as it attracts people to want to join your group. If you pick a focus that's fairly random and obscure, for example a team based on a specific weeb anime, it will probably die very quickly. It should be something a large portion of the player base can relate to.
At the end of the day, as corny as this sounds, I think the most important thing to leading a team is getting to know each (or as many as possible) of your members personally and treating them as a friend first and a team member second, or at least attempting to (some people you will encounter will be inherently quiet or rude - there is no avoiding that). I think the best team experiences in PokeMMO are when the team is a collection of friends who can for the most part get along with each other and joke around. Since we have no real team based features in PokeMMO (other than Team Tournament), the only real other functional purpose of teams is as a private chat for friends to goof around in and have a good time with one another. If you can do that the rest should come fairly easily, as long as you don't mess up horribly.