Counterspelled Booming Blade

[Amnestic]

Fair point. In the case of my table, it was jumping down (like parkour) vs the monk's slow fall. All characters can jump, and the vertical jump doesn't specify that you have to jump "up". But the DM told me if I was jumping "down" it would be like the monk's feature.

I think there is a difference between an action approximating something a different class feature can do, and straight up rolling to have a class feature. Like I don't think you can roll to gain weapon proficiencies, and I don't think you could roll to gain spellcasting.

But if the effect is the same then what's the difference?

You said "jumping down (parkour)" vs. "Slow Fall" when the end goal is the same: Reduce fall damage. One just has a name on it, because it's "always on". That was the revelation you hit: It's "okay" to emulate a class feature with an ability check, because the class gets it all the time without rolling.

So how's it different to "unleashing a burst of magical flame" vs. Fireball? Is it purely vibes based? If spellcasting's off the table...Wild Shape? Channel Divinity? Stunning Strike?

Nope. Check Chapter 7. You only make an ability check if something is possible.

Why isn't it possible? Where's that written?

If I can emulate a superhuman feat of agility by succeeding on a skill check, why can't I emulate a supernatural feat of magic by doing the same thing? The world is magical, after all.

Would the answer change if I were a race that had an innate cantrip such as high elf or tiefling?