This bothered me today when I discovered it - I miss the option to switch from a post reply to a PM.
The particular use case was a public post asking for details to be sent in a PM, but I know I use this kind of thing all the time and I miss it.
It made quoting to a new PM easy - especially here on Meta where I might ask for sensitive information in a PM while quoting the public post.
Related: it is now difficult to start a message via the keyboard.
I would have expected gm+c to do so, but that didn’t either.