Aesthetically, very similar, just some small improvements.

In terms of performance, features, etc… It’s way better.

I’d say that there are some differences, but whereas Plasma 3 to Plasma 4 design-wise broke the mold, going from 4 to 5 was seemingly less of a major version milestone and more of a major version touch up.

I’ve used both, a long time ago, and I might be off base here when comparing Plasma.4 with 5, but what’s also good about the transition from 5 to 6 is something called skeumorphism, i.e continuously developing a design language rather than making ground breaking changes. Plasma 6.1 hits a little higher mark, but considering all the UI work KDE has gone through the last year, it’s time to reap the rewards and hard work of the community.

That being said, backend wise, a ton of new stuff has come and even gone. The last years there’s even been an attempt to rewrite a lot of old stuff into modern code, in preparation for Wayland support in somecases- which was a doozy. Wayland is not an easy protocol to implement - or so I’ve heard.

We lost the cubic desktop, and got it back again, it’s been a wild ride. Kudos to the Plasma team for finally starting to catch up to GNOME in the UX department and that their efforts have been fruitful. I’ll be trying out Plasma 6.1 as my daily driver once it is released as stable.