In relation to the Plasma Widget Explorer, I believe that the arrangement in two rows corresponding to the search field and button categories, squanders much vertical space. By contrast, the Plasma Activity Explorer solves that problem very well, integrating all in one top row.
The same solution should not be adopted in the Plasma Widget Explorer?
Here a simple mockup to illustrate the proposal:
https://drive.google.com/file/d/0B_nP-bvAo4mtVkstXzJlMHB1TUk/view

It would be nice to have similar layouts between the different elements yes and the activity ones is very well designed now.

+1