The situation is that a CLA is still required.
Personally, I don't care (I'm just a consumer who is not affected by the CLA). Others care and insulting them as "People who don't know any better" is not right.
They can use the key. They just don't change default keyboard shortcuts during a release cycle (first generation Plasma precedes both Unity and Gnome Shell).
The CPU thing is either a bug or file indexing is not yet complete (although that should not take more than a few hours).
How much RAM is utilized depends on the components you loaded. "Convenience distros" tend to start more services by default. The KDE Workspace is very modular. You can disable/remove many things, esp. if you compile yourself.
That said, it's not a workspace primarily for low-end computers. If it requires more RAM than your PC has, maybe look at LXQt.
The focus for Plasma 5.0 is -- for the most part -- the port to Qt 5, not adding many new features (although there are a few new ones). Most new features will appear in subsequent Plasma releases.
Canonical never contributed much to KDE. Yes, a few bits here and there but overall mostly negligible.
Blue Systems OTOH is a big KDE contributor. Things like KDE Search improved a lot thanks to Blue's involvement.
As for Kubuntu: It appears to me that mostly the same people are involved with it. The only KDE thing mostly specific to Kubuntu is Muon (and that's being ported to Debian as we speak). Everything else by Blue Systems is AFAIK cross-distro.
The Kubuntu guys already announced that Canonical's own display server Mir is not an option. Therefore it remains to be seen how feasible Kubuntu remains with Canonical pushing a Wayland competitor.
Actually more optimization is taking place than visual refreshes. Porting the stuff to Qt5 (esp. QtQuick2) should speed everything up on non-ancient PCs.
Visual refreshes are just easier to explain. ;-)
It's mainstream in the sense that the Gnome community fragmented so much in recent years (see Mate and Cinnamon "spin offs") that these days Xfce has roughly the same mind share as Gnome -- if the recent poll on Slashdot it any indication. http://slashdot.org/poll/2751/which-desktop-environment-do-y...
The redesign is not in this beta and, depending how long it takes, maybe won't make it into the first final release either.
Partially it's even by design. It's not intended that Plasma 5 diverges too much from the current release to ease the transition.