Actually, I'm using btrfs for the root and not for vm disks (for them I use lvmthin instead). I use for integrity and autorepair (with raid1) and in the future (shortly) also possible snapshot of the root for faster "rollback" in case of serious issue after system upgrade.
For the root part I also think it would be useful to have the root in a separate subvolume and for another thing that I know for now that it could be useful to have in a separate subvolume also /var/log so that you can take snapshots of the root without including local-btrfs and without the logs (both for less writing on the snapshots and occupied space and in case of restoring the root keep the logs is useful for debugging).
Are there any other sub-volumes that you think would be useful to have?
I saw that developers added compression support for btrfs in the installer (configurable) for the next version (from fast look to pve-installer git), another useful thing, maybe by reporting well what could be useful for the sub-volumes so they could improve that part too
