Not that there's anything wrong with open hand either - it's all about the right tools for the job - but we are talking about punches specifically here.
Not really sure if this is a striker's thing or just a particular guy or two I've sparred with, but there seems to be a tendency to cant the weight forward in people training for strike-only sports, which can leave you vulnerable to offbalancing/takedown/throw.
Having read some texts on bare-knuckle boxing, where in fact takedowns were allowed, I have noticed some commonalities in advice - minimize tension, keep upright. Using the shoulders for defense is also advocated in both.
Having only boxed amateur, I can't say whether this holds true for the pros, but in systema I've been trained for hitting with all parts if the fist, whereas in boxing it was knuckles only.
Also, I know from speaking to others that even modern boxing varies a great deal by region. A friend from Uzbekistan (who also does systema with me) used to box there, and he says in the eastern republics, hooks and uppercuts are used much more than you see in most fighters in North America. He also showed me a hip-toss disguised as a punch (holdover from bareknuckle days?).