I won't pretend expertise, but I have some HEMA experience, admittedly in parallel to SCA fighting, which has some different techniques due to different rules. My Kenjutsu credentials are slightlier shabbier, in that I did my training in informal classes at work, admittedly from someone who had proper accreditation with the local schools. I actually found my prior experience was a bit of a hindrance because of the difference in styles. I'll provide my input, in advance of more experienced people offering help.
A lot of it comes down to the sword
Other than the length, being edged weapons, and generally being wielded with two hands, the swords have several differences.
- The katana has a single edge while European longswords generally has two.
- The katana is traditionally curved, and more tapering while longswords are straight and tend to be the same width for most of the blade.
- The katana is sharp along the entire length while the longsword generally has a dull part at the bottom.
- The katana has a more abbreviated hand-guard, an oval guard, compared to a European longsword which will range from a simple cross-guard (arming sword) to a more comprehensive basket hilt.
- Katana generally do not have a pommel and have a longer hilt
- A katana is a bit less tip-heavy.
The first two qualities of the katana basically come down to one major issue, that Japan did not have access to high-quality iron. As a result, they used a folding process to create the katana, which resulted in a better quality of steel compared to what they started with, but still relatively brittle. Thus, the blade was blunt on side, to reinforce it, and the blade was curved to distribute the blow over a longer amount of time.
You don't chop with a katana... for long
Because the katana is more fragile, you really don't so much swing it into your target like a bat so much as that you make contact, and then you move the sword to slice your target. It takes a lot of technique to do this properly, and at higher levels, especially against a stationary target, it does look like a chop because you're doing one fluid movement where the slicing is done in conjunction with the contact.
This kind of goes hand-in-hand with the armor situation. Again, there was a lack of good iron in Japan, which meant that armor moved towards a lamellar design of multiple laced scales, often of leather, rather than the broader plates of metal favored by European medieval warriors. Part of European swordwork involved heavier blows because it was the only way to damage someone through heavy plate, striking them hard enough to dent the plate or to cause internal trauma.
You have a flat side on the katana to brace with
Admittedly, this is also something done in European longsword when you have gauntlets and vambraces, but it is more common in Kenjutsu for some techniques to use a free hand or forearm to brace your blade or to move it in a situation where you've become into too close of quarters. It does also mean that you also have to consider which direction you're cutting with.
Katana do more deflection than they do direct opposition
Again, down to fragility and differences of weight, there is a stronger emphasis on deflection with the katana rather than blocking with the edge (an exception is made here, of course, where you can arrange to block an unarmed strike with your edge). With the longsword, I was taught to block with the edge because the cross-section of the sword was strongest in that direction. With the katana, I was told a direct block would significantly chip, and weaken, the blade.
As more aspects come to mind, I will add them.