The mechanics are nowhere near as different as the focus and training. A Boxer trains to punch and only learns about fourteen techniques, that's a limited toolbox but the beauty of that focus is that a Boxer trains to make himself/herself incredibly good at using those tools. In the early stages of training 0-4 years I would put my money on the Boxer in most cases, after four years a person who has practiced Boxing is intimately familiar with their whole arsenal while the Karateka is still learning technique and training to make their body capable of performing many of those technique.

It's a real case of quality over quantity. After the four years of hard sparring I'd put better odds on the Karateka, they've usually become good enough with their more basic techniques to survive the match long enough to identify and attack any of the Boxer's weaknesses.

As Max said, boxing focuses on a few techniques and does them very well. But this is a unfair scenario. You put anyone in the ring against a boxer using boxing rules, the boxer is of course going to win. That's the way it is.

Another factor is sparing. Karate artist do not spar nearly close to the amount that the boxer guys spar. That is a huge advantage. However, if it's a no rules fight, the karate guy could win. Low kick the boxer and stay out of range and he'll lose.

there is not one style of punch for either art but if you go general, the main difference would be in the gloved vs ungloved hand. One mistake that people make is thinking that Karate only uses the straight punch seen in tournaments but if you look at the Kata you will see hooks (Pinan Godan) upper cuts (Pinan Nidan), and many different hand strikes for the variation of body targets. In Traditional Karate (Okinawan) you are taught to use the right tool to break the right target without breaking the tool.

Another mistake is that some think that the way tournaments do punches is how Traditional Karate is supposed to punch. Check out a real Karateka (only those who use makiwara are true Karateka based on the old masters) punch you will see the usage of the entire body as does a boxer. The main difference is in the gloves.

When we now look at had fractures, boxers tend to punch in a way that when they do not have gloves will break their hand, Clinically, there is a fracture called a "Boxer's fracture" The other problem is that modern boxing is won by points while traditional Karateka should train in Ikken Hissatsu

How is the mechanics different? I sometimes hear karate people say that straight punches are technically better than a boxers punch but yet I also know that boxing is one of the strongest skills you can have when it comes to punching power and speed.

So I guess the question is if a boxer and a karateka "sparred" with the intent to harm, but the karate guy was restricted to using only punches and blocks, would one have a major advantage over the other, all other factors being equal?