Those are not radically new features, but rather a refinement of what it was available in KDE Plasma Desktop 4.3
When the mouse cursor is over a task, the corresponding window gets highlighted (dimming all the others), this works for groups too, highlighting the goup’s windows. This was already available in 4.3, but it’s works in a way more reliable way now.
What’s new is that now moving the mouse cursor over the taskbar tooltips triggers the highlight effect too, useful to quickly pick a window from a group, since the thumbnails are now clickable too, activating the proper window.
Another new and very useful feature (even if a bit “hidden”), is seen around the middle of the video: if you click on a task group with Ctrl pressed, it doesn’t open the usual popup menu, but it triggers the present windows effect, giving you a fullscreen chooser of the group windows
Tasks should have text or not? The text in the taskbar is a really important disambiguation, so should always be shown when possible. but in some cases it’s just messy because there is not enough room. so now with vertical tiny taskbars or when there are really many tasks the text is authomatically disabled and enabled as soon as possible.