OK... this is deeply concerning. Especially with with new AstroPrint-Touch coming out... YOU MUST PUT IN AN ESTOP!
All industrial equipment used in consumer/professional environments require an eStop capability. The current cancel print feature is more like, "oh let's make sure, then we'll get around to it after the next sequence of G codes are done."
Today I had a print peel off the table and jam the machine. I was monitoring remotely and I saw via the camera the print-carriage was jammed on the ball of PLA goo. I could hear it banging from 20 feet away behind a closed door -- I issued a cancel print. It took almost 30 seconds before the printer stopped as it was clearly waiting for the a command sequence to stop. It appeared it was doing several batched G-codes.
Since I was just down the hall, I ran into the printer room and turned off the printer. Figuring the astro box would time out, I turned the printer back on about 30 seconds later and it returned immediately back to running and jamming again...
All the other S/W control packages (Simplify3D, GCodePrintr, Mach3) pretty much do an immediate motor off when the stop button is pressed.
What if a kid or overly curious person got a sleeve caught in the machine? And cannot reach around the back of the machine to power it off.
PAUSE is not the same as eSTOP. That is just a "feed hold" operation. eStop means you cannot just resume, you have to figure out what is wrong and fix it.
So... PLEASE PLEASE PLEASE put in an all-motor off estop button. In fact, while you are at it, please add the ability in the S/W to select an I/O pin to allow people wire in an external e-stop switch. (Normally ON is typical in the industry as per how eStop switches work, so you can wire several in series if you need to).
I am stunned you guys only "think this is a good idea", and "we need to think about the UI".
This reminds me of the joke, "the speed limit of light at 186,000mps is not just a good idea, IT's THE LAW".
Honestly, by law you must put in an eSTOP capability, to shut off all movement (axis motors and extruder at least) during an eStop. I'd recommend turning off the heat too.
Seriously, you are opening yourself up for severe liabilities. Disclaimers won't get you far if someone is hurt.
I look forward to seeing a proper eStop in a build very soon...
Did I mention the word "please"?