The new C button for GameChat, and a general increase in its size compared to the original Joy-Cons. There will also be a significant change to the vibration , which some players may find polarizing.
According to a game description on Nintendo's backwards compatibility page (as reported by VGC), the Joy-Con 2 controllers will see a volume reduction in their vibration noise. The description for Everybody 1-2 Switch reads: "The volume of the Joy-Con 2 controllers' force vibrations has been reduced compared to the Nintendo Switch Joy-Con controllers, so it may be more difficult to find hidden Joy-Con 2 in the minigame Joy-Con Hide & Seek."
HD Rumble Is Being Reduced In Switch 2 Joy-Cons
A Small But Probably Noticeable Change
The reduction of the HD Rumble volume for the Joy-Con 2 controllers is a small change, but one that is likely going to be noticeable. Having HD Rumble allows for a greater variance in rumbling than standard vibration, and it has been used effectively in past Switch titles. In Super Mario Odyssey, the Joy-Cons would vibrate more intensely the closer you were to a hidden Power Moon, for example. The Switch version of Skyrim also made good use of the feature for its lockpicking mini-game.

Nintendo Switch 2's C Button Will Be Locked Behind A Paywall After A Certain Time
The C button, used to launch Nintendo's GameChat feature, will essentially be useless to those who do not want to pay for a Switch Online hip.
Luckily, the HD Rumble will still be present, just in a reduced capacity. The lower volume and presumably less intense rumble might make some fans happy, but others could feel like immersion will suffer because of the change. Either way, the HD Rumble may not be the biggest concern for Nintendo fans. The Switch 2 may address the notorious stick drift of the original Joy-Cons, but it remains to be seen if stick drift issues will continue with the Switch 2, since the sticks won't use Hall effect technology that could avoid the problem.
Our Take: The Joy-Con 2 Controllers Seem To Mostly Be An Improvement
There Are A Couple Of Drawbacks, Though
Overall, the Joy-Con 2 controllers are shaping up to be an improvement over their predecessor, assuming everything functions as intended at launch. The new Joy-Con 2 mouse functionality will add new ways to play some games, like Metroid Prime 4: Beyond. The increase in size will be a benefit for people with larger hands or when using just one Joy-Con for multiplayer games, and the new magnets seem to be quite strong for firmly gripping the controllers to the console in handheld mode.
That being said, there are a couple of things that would have been nice to see, like analog triggers instead of digital triggers. The news that the HD Rumble volume will be reduced for the Nintendo Switch 2 controllers might disappoint some fans who loved the louder vibration of the Switch 1 controllers, but just how much the HD Rumble vibration sound has been reduced remains to be seen.