The best microwave UI I've used has several functions above the number pad, all of them completely optional. Typing a time on the number pad and pausing made the microwave start -- to rephrase, if you press 4 5 and didn't press anything for 3 seconds, the microwave would begin and operate for 45 seconds. Pressing 1 3 0 and walking away would run it for 1 minute 30 seconds. When it was done it beeped 3 times, then once every 10 minutes, until you open the door or press a button. At the bottom was a Power button which you could press to change the power level at any time, and a Start / +30 sec button which would add 30 seconds to the amount of time on the timer. And if the clock wasn't set, it would just display 00:00 solidly, not blinking.
Maybe this is a mis-remembered mix of the best UI elements from several microwaves, but to me it's ideal.
I'm also not a big fan of your "walk away while the microwave is off and it turns on after three seconds" model.