> This is a challenge with the iOS (and Android) UIs, I think. Mice and trackpads have a more obvious and somewhat richer set of "verbs" for interaction
GTK+3 has a good approach there, I think. It turns the window titlebar into a thick "header bar", which includes a "title" part for easy grabbing as well as a handful of button-accessible menus (fewer than the top levels in a traditional menubar), including a kitchen-sink "hamburger menu" for lesser-used options. It becomes a bit less convenient for mouse&keyboard use, but the touch usability is absolutely there. Too bad that running Linux on tablet-like computers is still way too fiddly, it could be a fierce competitor to the iPad ecosystem for more pro tasks.
Added: And on recent GTK+3 releases, one can shrink the window horizontally and the buttons will simply shift to the bottom when there's not enough room for them in the top headerbar. This solves a flexibility issue with the previous headerbar-only approach (especially on smaller or lower-res screens), while still being quite intuitive.
It has hjkl, right? What more could you need?