In the edit menu, I would find it useful to have an Invert Selection function. So for example I could select the lowercase letters, select Invert Selection, and then all glyphs except those lowercase letters would then be selected. I’d find this useful when I want to delete, or run a filter on, most but not all glyphs.
I second that. Was just going to request an Invert Selection function for the edit view.
+1 with a shortcut it would rock, but is there any available shortcut yet? 
What would be selected if you invert in the edit view?
If some node are selected, should be the rest of the path selected or everything in the layer (guides, components …)
Just the rest of the path and any other paths, I would say.
Is there an invert selection function for the Font tab yet?
Here is a script for those interested:
http://www.glyphsapp.com/forums/index.php?action=vthread&forum=2&topic=2002#msg9985
In FL there is one, yes. I would to have one in Glyphs.