meta is ctrl if you're using Windows or Linux, and command if you're using OSX.
Note that most of the keyboard shortcuts are disabled when you're focused in a text box.

• ? toggles this reference.
• spacebar expands or collapses the children of the currently selected node.
• up, down move the node-selection across the nodes in vertical order. 
  shift+up, shift+down move the node-selection across the siblings of the currently-selected node.
  left, right move the node-selection up a level (to the left) or down a level (to the right) in the document tree.
  
• Button on bottom right of screen to switch between read-only mode and edit mode ("localauthor").
  u (for user mode) does the same thing.
• l to load or reload the file selected in "Manage imported files..."
• s for search, shows all occurrences of references to the currently selected node.
• z for zoom, hides all nodes except for those in the currently selected node's subtree.
• meta+esc when a node is selected removes the selection.
  Clicking the node does the same thing.
• Author mode-only keyboard shortcuts etc. Some features not documented here yet.
  • i toggles inline editing feature.
  • m toggles showing mutids (for in-text references).
  • Moving nodes:
    meta+shift+down moves node up.
    meta+shift+up moves node down.
    meta+shift+left moves node left (changes parent).
    meta+shift+right moves node right (changes parent).
    To rearrange nodes with the mouse, click and hold on the icons.
  • return inserts a new text/html node as the next sibling of the currently selected node.
    shift+return inserts a new text/html node as the first child of the currently selected node.
  • ctrl+r will refresh the entire document tree.
    r alone, while a node u is selected, will refresh only the subtree rooted at u.
    (Some things in the graphical interface are not yet set to update immediately when the underlying data changes, or the updating is temporarily broken.)
    
  • esc when in a typing a position removes the cursor from the typing position and selects the closest node.
  • p (for panel) when a node is selected opens a panel where you can edit any of the data associated with that node.
    Clicking a node's icon does the same thing.
    shift+p closes all panels.
  • delete when a node is selected deletes that node.
  • meta+c sets the currently selected node to be copied.
    meta+v while node x is selected, after using command+c on node u, pastes a copy of u as the next sibling of x.
    meta+shift+v is like command+v, except instead of pasting the copied node as a sibling of the currently-selected node x, it pastes the copied node as a child of x.
    
  • The undo/redo buttons at the bottom left of the screen should be self explanatory. They may be buggy though.
    meta+z and shift+meta+z are the shortcut for undo and redo, as usual.