I am using two screens, and I used to have kicker over the two displays. Can I do that with Plasma?
No. The reason is that having a panel over two displays adds a great deal of complexity, especially when the two displays have different resolution. As a result of this added complexity, this feature would not be guaranteed to work in all cases, hence it was not implemented.
Can I put the panel on only one screen?
All Plasma panels live in one screen. If you want panels on multiple screens, you can add panels and drag them to your preferred location using the panel controller you get when clicking on the Plasma icon in "Unlocked" mode.
On multi screen setups, the ZUI operates on all screens, the Dashboard just on one. Why is that?
The workflows are different. When zooming out, the user wants to get an overview, therefore activities on all screens are zoomed out. Whereas when using the Dashboard, the user usually wants to access specific functionality and might not want to interrupt his or her workflow on other activities.