Exactly the same things that would happen if the game was 60FPS I would expect.
It's pretty noticable.
Play a game on PC like Destiny 2 in 60fps then immediately hop over to Destiny on console and the difference is night and day.
If you play a ton of 30fps games your eyes adjust to seeing 30fps just fine, but once you go from 60 back to 30 is when it's insanely noticeable.
Some games do 30fps just fine though, usually platformers or anything not a shooter thats in third person looks generally fine in 30fps.
FPS or TPS look and feel way better to play at 60 though. I'm pretty sure you played Metal Gear Online 2 and 3 right? MGO2 ran at like 20-30fps and generally felt pretty slow even though it was the better game by a mile. MGO3 though felt BUTTERY smooth compared to MGO2, could you honestly not feel that difference?
If anyone feels like this and wants to test it out, I recommend playing Rainbow Six Siege on console. Play one multiplayer match then switch to Terrorist Hunt with v-sync enabled. The gameplay feels INSANELY slow in Terrorist Hunt because it lowers your FPS to 30 with V-sync enabled.