When using SPACE or ENTER (return key) to repeat a command, it should repeat the last used command except for the "undo" command. So many times I want to undo what I just did and repeat the last command, but pressing the space bar or enter key just repeats the undo. The cases where you want to repeat "undo" are very few, and there is already a great universal keyboard shortcut for that.
Example: I am creating a new work plane defined by 3 points. I accidentally select the wrong 3rd point, and I want to start over. I press CTRL+Z to undo that plane creation, then need to select the workplane command again. It would be so useful if I could just press the space bar to start the last command that I used before the undo command.