Here's my perspective as someone who entered UPT with few hundred GA hours (CPL w/instrument rating) and currently a T-6 IP.
For starters, ground ops (walking out to the jet, the preflight, getting the aircraft started and taxied) sound benign but in reality, the AF wants them done quickly and accurately. You won't have time to sit under the canopy in the heat or cold and go through the checklist item by item, especially when you're in a formation and you have a VHF check-in time. You'll need to create flows to make sure everything is in order and it helps to do things the same time, every time, so something will hopefully seem off to you if it's forgotten. The struggle between speed and accuracy is what some students struggle with, to include those with previous flying experience.
One large difference in UPT is the vastly different traffic pattern. In the GA world, you're used to a single rectangular box pattern. In UPT, you have several different patterns (overhead, straight-in, low pattern, high pattern, breakout/reentry, etc.) controlled by the RSU within the Class-D and things happen fast. You'll have close to a dozen other T-6s moving at 200 knots at its busiest, and you have to listen on the radios for every little detail while precisely maneuvering the aircraft. It's not uncommon to routinely pull several Gs in the pattern to follow the precise ground tracks. It takes several flights to build your SA bubble enough to be comfortable to solo... this is something you can't really prepare for in the GA world. It's also a blast once used to it.
The formation phase is obviously entirely new to most. As someone who occasionally flew formation with buddies in the GA world before UPT, it was nothing like military formation. For most with prior hours I'd say, this is where the playing field is generally leveled. I was a strong student in contact and instruments, but perfectly average in the formation phase and it was similar to learning to fly all over again in some ways. How so? Like in the pattern, things happen fast and you're maneuvering in relation to someone else while keeping up with precise, timely, and correct comms. As lead, you need to make the appropriate decisions for the formation and you're constantly under pressure to do so. But again, it's also a blast once used to it.
Also, unlike on a GA training flight where you just fill the fuel tanks and typically don't have to worry about fuel, you will always have a set amount of time/fuel to accomplish each sortie in UPT. You simply won't have the time/gas to mess around. You need to be quick to accomplish your profile within these constraints, whether single ship or for the formation. It's added pressure.
Those are just a few specific examples and I'm sure others will chime in with more. What helped me the most in UPT was having good stick and rudder skills from GA flying and my instrument rating. This allowed me to fly/trim the aircraft so I could focus more on what was going on outside and build an SA bubble. Previous flight time will set a foundation in terms of some basic general knowledge and hopefully some stick/rudder skills, but you will have to much to learn in terms of AF procedures and applying them while inverted/under moderate G/potentially being yelled at by the IP. Did I mention that it's a blast?