Re: Taking a punch...
^ As stated above, it's what you do with the time you've got to react.
The punches you don't see coming hurt the most because you will not be bracing for impact, possibly moving into it to make things worse.
There is bracing for impact and then there is moving with the punch - the slightest movements can take much of the sting out of it.
Underlining this all is your mental strength. If you want to win at all costs and the punches are not shaking you, you'll force yourself to stay on your feet. A lot of knockouts are from fighters that have decided it would make sense to submit, or the sense of urgency to get away from punishment became too much.
Neck, jaw muscles and your centre of gravity all play a big part.