I agree that it is currently hard to locate effects and it would be great to improve this. Large size of the Segment editor module GUI is a problem, too, because it forces the user to scroll a lot. We would need layout improvements that address both issues.
Introducing rows and large category labels would help with finding effects, but it would uses much more space, so we would need to scroll a lot more.
I quite like the vertical toolbar shown here. We could probably have two columns, if needed. It would be a regular toolbar - no flow layout, button labels hidden by default (very easy show/hide, just a flag).
is ribbon interface a possibility with QT? If so, I agree it makes sense…
Not really. We could use similar high-level layout as ribbon controls, but tabbed widgets and groupboxes are just a very small part of the whole ribbon concept. I like the ribbon interface and it would give Slicer a modern look, but it would require a lot of work to be implement and it should be then used for the whole GUI, not just for Segment Editor.