Some cells "do stuff" without producing output. If "doing stuff" either has a non-negligible chance of silent failure; "run forever" risk; or "takes more than 3 seconds to complete" some kind of visual queue as to the state of the "executed" cell would be valuable. In a normal notebook view the "prompt" on the left side performs this duty: blank means un-executed, star means "executing" (though possibly stuck), and a number means "executed/done". I don't know exactly what the right answer here is, but an idea:
play button to indicate a cell can be executed (and clicking this will be the same as SHIFT-ENTER)
hour-glass in place of star to indicate "executing"
perhaps with a mouse-over effect of (X) or ([]) (X or Square button) which will cancel execution of that cell
Some cells "do stuff" without producing output. If "doing stuff" either has a non-negligible chance of silent failure; "run forever" risk; or "takes more than 3 seconds to complete" some kind of visual queue as to the state of the "executed" cell would be valuable. In a normal notebook view the "prompt" on the left side performs this duty: blank means un-executed, star means "executing" (though possibly stuck), and a number means "executed/done". I don't know exactly what the right answer here is, but an idea: