• Good design techniques (uses a lot of design patterns ;)
• wysiwyg: It's rendering all the time using OSG, so what you see on the screen is what you get when you open the scene with osgviewer or your own app.
• You can see a tree representing the graph where you can select nodes for all the operations. This tree is really a graph, the same node can hang in two different branches, and it's a shared node, not a copy, changes on the node in one branch affect to the same node in the other branch.
• Each node type has a properties dialog integrated in the window (most of them are already supported).
• Of course, you can also manipulate transforms visually, there are tools to move, scale, and rotate transform nodes visually with the mouse.
• All the operations except for the new/open/save and zoom changes, can be undo and redo without limit.
• You can create new group (Group/Transform/Switch/LOD) nodes (It doesn't allow for creating of geodes, it's not a modeller, only a scene composer).
• For these files that only contain one geode, you can also insert a Transform node as a parent of the selected node, even if it's the root node.
• You can open all the formats supported by OSG and also save all the formats supported for writing by OSG.
• There is a tool for merging in files, you can add child nodes that are readed from file (this is very important if you are going to compose a complex scene from several models :)
• Nodes can be copied, pasted into group nodes, and even cutted.

If you want to suggest new features that would be usefull for you, please use the mailing list