Many respond with specific issues, but for me it’s just the federated structure. For non tech persons this may be a difficult concept to grasp and even though I programm a lot, some things got me confused, because I’ve never used anything like it. List for readability:

• if I remember correctly I had to choose one instance for my account. (I had no idea if I get any restrictions depending on what instance I choose)
• How can I visit or follow another instance and more importantly where can I find a list of instances? (I am using lemmur, but this may be a general point of confusion)

Maybe I missed some obvious things, but a getting started video would be great :)