>>1008242
Almost everything. Physical location of IP, system brand and specs (cancer JavaScript scripts), data being transmitted to and from server and client, etc. I'd encourage you to look more into ARP spoofing if you want to learn more about MAC addresses, ARP and high level MITM attacks.
The entire OSI model from layer 1 to 7 and all most of the protocols are susceptible to man-in-the-middle and spoofing attacks, etc. You have to remember that the OSI model was created and implmented decades ago when protocols that transmitted data in plaintext was the norm. The icing on the shit-sandwich is the modern day 'web development' languages like JS and PHP that can fingerprint you when you visit a site. Most of the protocols today have SSL/TLS bolted on, which works for the most part, but E2E would be more preferable for security and some anonymity if we lived in a perfect world, but we don't.