First off, these are some really nice improvements.
I fully agree with the first two points Jostein made.
It might even make sense to have a small space between the built-in categories and the custom ones. And I’ll also tell you why: When teaching a beginner’s class, students usually do not have any custom nodes installed. So for them it would be quite disorienting to see a different menu structure in the projected image as compared to what’s on their screen. The same goes for video tutorials, I guess. (Also, in Revit, the addins always come behind the built-in toolset…)
Regarding the third point Jostein makes:
Personally, I like the fact that there are icons for each subcategory like points, UVs etc.
It seems like categories and subcategories are displayed differently depending on how deep their children are nested. If you look at the Geometry category, for example, the last subcategory (Tesselation) is displayed as an expandable list whereas everything else is displayed as an icon. I can see this making sense in a lot of ways, but I also find it a bit confusing. Don’t know if there’s a way around this, though.
Some other random observations on the new library:
The library view looks much cleaner now. I like the new condensed look - there’s a lot more space now for custom categories.
I like the fact that the categories are now organized alphabetically.
As already requested on GitHub (https://github.com/DynamoDS/Dynamo/issues/3721), you should provide package authors with an easy way of providing custom icons for their packages (for custom nodes as well as for custom categories).
It would also be nice to be able to turn icons off altogether (View or Settings menu).
This is not per se related to the new library, but: When browsing through the library, I realized how many built-in nodes lack a description. This is something you should definitely put some work into. Especially for new users those node descriptions are essential.
One thing I liked about the old library view is that items that only had one child would always expand directly (the Office category is a good example - clicking on Office would also directly open the Excel subcategory). It would be nice if that could be brought back.
Oh, and I really, really like the fact that you are labeling custom categories that are named Create, Actions and Query just like in the built-in categories.