Just a couple of things missing....

Overall, I quite like this app. You get a visual representation and there are a couple of tools and handy functionalities.
However, there are also a couple of things that are bugging me:
- The UX and the design. This app would greatly improve if the author would ask an interface and/or graphic designer to help out.
- The JSON text (lower pane) is not editable
- A search box: Theres no way to filter the current JSON list
- Selection goes away when an entry is deleted (need the mouse to continue); these are basic list editing features that should really be handled with care.
- Statistics: Theres no line- or itemNumbering, no info how many items there are
Nevertheless, IMHO its the most advanced app regarding JSON content in the Mac App store at this moment.

frequenzdetektor about Cocoa JSON Editor, v1.1.2