Unique ability for each class
In Diablo 4, characters can't attack monsters or joust while riding a mount; however, when approaching enemies, they can use a powerful dismount attack [1] that's unique to their class. This involves jumping off the mount into the heat of battle and casting the ability:
"We've added dismount abilities. Each class will have dismount abilities that fit their fantasy perfectly. If you're a Sorcerer and you just want to get off your horse like normal, you can still do that, or you can turn into an ice storm and freeze everything in your path."Tiffany Watt · Diablo IV Senior Producer
Barbarian slams the ground
In video footage shared by Blizzard, we see a Barbarian leaping off his mount with his weapon high above his head and pounding it on the ground, causing mass devestation to the surrounding monsters. Some monsters are severed in half and splattered.

Druid goes werebear mode
The Druid turns into its werebear form while dismounting, smashing enemies with its bear claws/paws when it lands. This state seems to be short-lived, but the Druid can do a lot of damage while in it.

Sorcerer casts an ice storm
Upon jumping off her horse, the Sorcerer turns into a ball of ice and hurtles towards the monsters on the battlefield, freezing and knocking them over. A wide path of ice and cold sets in around the monsters.

Rogue sprays a rain of arrows
As the Rogue dismounts, she aims her bow into the sky and down rains dozens of arrows onto the monsters in the battlefield in front of her, many of them dying.

Sources
Article | Source | Date | |
---|---|---|---|
1. | 2019 BlizzCon: Diablo IV Unveiled | Blizzard |
Changelog
Ver | Change | Date | |
---|---|---|---|
Alpha | Added section on Druid dismount ability | ||
Alpha | Published original |