I think there are some things that could have some improvements.
1. Add a Title over the Materials list, so its clear that the Load, Save and Delete buttons are not meant for the materials.
2. Add a Delete button to the buttons below the list to remove the selected material from the Terrain Materials List.
3. Maybe hide the Materials List when in not in Paint mode, so you don't get confused and think you would paint the Material instead of changing the Terrain. Maybe there is even another way to make it more obvious in which mode i am currently in than just that little dropdown.
4. Maybe a Preview of the currently selected Material could help in knowing what you would paint currently?
5. I think i mentioned that somewhere already, but shortcuts to change the Tool Radius / Hardness etc. are really useful. Maybe like Photoshop and many other drawing tools do it. like holding CTRL + Left Mouse button and moving cursor up/down to change Radius, CTRL + Left Mouse button and moving mouse left/right to change Hardness, CTRL + Left and Right mouse button to change strength. (I would have to check a drawing tool again to see how it is exactly done there, but its pretty intuitive since you also see the cursor change depending on what values to modify this way.
These are just some ideas that i think could be helpful in using it.