I ride an old 70s schwinn 24 upgraded almost completely with new saddle, cotterless, and new wheel. None of those upgrades had any significant effect on the handling of the unicycle.
Today, I tried a Torker 24 in a shop. Same seat height, same crank lengths and Q, same wheel diameter, same tire width…almost everything the same. But it felt TOTALLY different. I had a very hard time riding it. What makes it so different and so much more difficult to ride, when most everything is about the same?
You neglected the soul. The 70s Schwinn has a soul, but the Torker doesn’t. 
There are many differences, some of which can be significant.
The Schwinn is built on a flexy, 2-half frame. The Torker has a one-piece tubular frame, which means it’s a lot stiffer. That should make it more responsive (and lighter), but not harder to ride.
You are used to riding the Schwinn and not the Torker. Depending on your unicycling experience, this could be a factor.
You may have left out some environmental differences. Were you riding on carpet? Was the seat height significantly different from your Schwinn? Was the air pressure in the Torker too low? Any of these can make a big difference in the way the cycle rides, and I’m sure the difference is in there.
Anyone who has ever worked in a bike shop knows how important tire pressure is. We never sent anyone out for a test ride without putting lots of air in the tires. Air pressure difference could be the source of your perceived difference. Tire size or type could be the other, the amount of rubber on the road is determined partly by geometry, and the width of a tire is nearly as important as tire pressure.
Did the floor have carpet?
carjug
I brought my uni to the shop, and I was switching between them several times to compare, so it didn’t have anything to do with the floor.
The tire was the same on both, and I had them adjust the tire pressure in the torker to about the same as the Schwinn–although only approximately, tested by squeezing the sides, so this may be the issue. Seat height was the same.
I’m pretty sure it was mostly do to what I’m used to, because I’ve been riding the Schwinn for years, and I’ve never tried any other 24 inch uni.
Even so, it seems odd that it handled so completey differently. I really didn’t like the feel of it. I went there to check it out for a friend of mine who wants to learn to ride, to give him my opinion of it. I’ll be going back there again tomorrow with him, and I’ll check the tire pressure more carefully.
Sounds like you’ve eliminated most of the factors I mentioned earlier, though you didn’t mention seat height.
Another possible factor is the tire. I was going to say your Schwinn has a so-in-so tire, but you’ve upgraded it. Differences in tire shape, as well as tire age, can be pretty large, especially on an indoor floor. Old tires lose their grip with exposure to UV radiation, and they slide around a lot more on a floor. The new tire will be more grippy.
Also, if the tire has a round cross-section this is best for general riding. If either tire has a pointy cross-section, or a center ridge, that will make it ride differently.
Lastly, if you’ve only been riding one unicycle for a long time, you have to allow for differences. The more switching you do from one to another, the less you will notice minor differences.