Your email gets spammed, and your embarrassing subscription list and reading history gets forwarded to your boss/spouse are things that immediately come to mind. Also your PM’s if you use those. Lemmy should really rethink its privacy posture. Of course you should never share passwords between sites, so if your password gets cracked, it won’t be usable elsewhere.

Lemmy currently doesn’t have private communities afaik, but if it did, those would also be compromised.