A while ago I decided to take the plunge by upgrading the 5-node Proxmox VE cluster at work from the 6.4 version (based on Debian 10) to the new 7.x version (based on Debian 11). Debian hasn't made a GA release of 11 yet so some thought it curious that Proxmox would release PVE 7.4 to GA.
How was the upgrade process? Proxmox VE is a very light-weight system and doesn't use a whole lot of packages so it was a very easy upgrade process. Historically, Debian is one of the most pain-free upgrades among the Linux distributions. PVE does have some advanced features but I'm not using any of them... no ZFS, no Ceph distributed storage, etc. All of my VMs use a qcow2 disk image file stored on an standard partition formatted with XFS. I'm not currently using a lot of containers (mostly VMs) so the switch from cgroups v1 to v2 didn't impact me. They do have good documentation on the upgrade process as well as a software tool named pve6to7. I ran pve6to7 and it found no issues. I backed up all of my VMs and upgraded the first host without incident and then did all of the other hosts one-by-one until they were all complete. I haven't noticed any issues post upgrade. There have been quite a number of updates since the upgrade but I'm guessing that is fairly normal as Debian 11 approaches GA release.
What's new in 7.x? Not a whole lot... at least not in this, the initial release. The big change is the move to a newer version of Debian... which I imagine was quite some work. I'd expect them to add more new substancial features in future 7.x releases. I do post a video below from them showing the main new features in 7.x so far. Enjoy!