I've been suffering from this one too so I'll share the most annoying part for me. I frequently have reasons to type out glob patterns generally in backticks. The interface for this is completely broken in the new WYSIWYG editor. Here's what happens when I try to type: `//`, things work normally up until the second "" at which point the "/" between them becomes gets bolded, which makes no sense, because you can't bold text inside of backticks. Then when I input the second "`" it becomes a monospace block containing `//` and the ""s have completely ceased to exist.
They also somehow broke the tab key for tabbing through users when you're trying to @ someone, after the first tab it just starts refocusing your input boxes rather than selecting different users.
Edit: Ironically, HN's built in markdown seems to understand asterisks, but not backticks, which leads to behavior similar to Slack's that I'm ranting about, I've replaced them with to make it clear.