Here's a quick and dirty spec for a JCR Explorer, feel free to expand/refine!
More or less in priority order:
Visualize the nodes of the JCR repository in a tree view, and/or in a search view with a search box at the top and a list of matching Nodes below.
Display all the details of a Node or Property: type, mixins, value, and later locking status, versions, etc.
Provide RESTful URLs to access each Node and Property.
Allow Nodes and Properties to be created, updated, deleted, and moved around the tree.
When creating a Node, allow its nodetype to be specified.
Allow mixins to be added to a Node later on.
Handle the JCR Session save/abort concept, where a number of changes to Nodes and Properties are only saved when explicitely actioning a "save" command.
Later, use OSGi-based plugins to edit Property values.