It's hard to identify really - honestly put & like ssullivan80 rightly said - people are opinionated in this respect.
That said, I'm a believer that training is pretty much the end all & be all of martial arts - by that I mean you can have all the dynamite you want in your fists, if you don't put the training in as much as the other guy, it doesn't really amount to much --- also guys you face might not have dynamite in their fists but might be gifted in other aspects your not - so it's all relative.
That said also - punching power can be trained too a degree like Frode mentioned with the proper combination of factors - some of which he mentioned.
But it also has to be acknowledged that some guys have physical attributes that make them hit harder than anyone else (provided they train enough lol) - there's a reason why everyone isn't a heavy hitter & why some guys stand out from the rest - like Earnie Shavers, Tyson, Foreman, Liston, Prince Naseem, Hearns...etc etc ----- but that's where the difficulty lies & why this question will keep popping up - was it a result of training, lifestyle and the way they were brought up - that gave these men their attributes or was it a result of genetics & simply being born with it.
I'm of the opinion that it was training, lifestyle & the way they were brought up.
EDIT: NLM - I wouldn't say Prince Naseem had shit boxing technique lol just very unorthodox.