The <x id="START_LINK"/>sharing system<x id="CLOSE_LINK"/> implies that some technical information about your system (such as a public IP address) can be sent to other peers, but greatly helps to reduce server load.
The worst-case scenario of an average person spying on their friends is quite unlikely. There are much more effective ways to get that kind of information.
The username is a unique identifier of your account on this and all the other instances. It's as unique as an email address, which makes it easy for other people to interact with it.
The threats to privacy with YouTube are different from PeerTube's. In YouTube's case, the platform gathers a huge amount of your personal information (not only your IP) to analyze them and track you. Moreover, YouTube is owned by Google/Alphabet, a company that tracks you across many websites (via AdSense or Google Analytics).