A brutal DM

What would the result of this be? Non-D&D player here.

A counter spell prevents other magic users from casting their spell. It can be used by a magic user to block a spell to various degrees of success. This DM didn't use it with any villains in the campaign, except once, to block a resurrection in the final battle.

He killed a guy's charector, then basically re-killed him right after.
- He didn't just re-kill him. He plastered over the stone that Jesus was trying to roll away. It's brutal, and it's what makes campaigns epic. If the last boss is a cake walk, then what's the point. I hope the party survived and the barb is welcomed by their god.
Trust issues, lol

The players were trying to bring their fallen comrade back to life (and back into the fight) but the DM opted to use an ability of the dragon to counter the spell, meaning dead dude stays dead.
- Can't they just cast it again?
- Trust issues is right. I wouldn't want to play with a DM who does that. This is not just fictional cruelty, to take a player out of the final battle just to rub in how evil and clever the villain is just feels unsportsmanlike to me.
  
  But I see there is a whole genre of posts that's all about suggesting awful things for D&D games because it makes for spicy social media content. Same goes for, say, the False Hydra, a monster that does not fit with the games mechanics and imposes a metagame threat that bypasses the characters abilities and resistances entirely. I hope it's all just memes at the end of the day.
As far as I can tell the group was trying to bring the barbarian back to life. But the counterspell prevented it. So barbarian stayed dead
In dnd when you go to zero hit points you go unconscious. You then have some chances where you stabilize yourself or die. If you’re healed even 1 hp during that time you become conscious again. If you die your character can only be brought back to life with high level expensive magic. Revivify is one of those spells and the DM cancelled it with counterspell. This consumes the expensive spell and they possibly might not have another ready to go leaving the probably high level barbarian dead. The player would need to roll a new character for the campaign. Since they’re fighting a dragon that’s powerful monster they’re probably pretty high level which means this campaign may have been going for months or even years

The players could have expected this: they've noticed that the dragon has lair and legendary actions. It's clearly dangerous, and it's smart. The dragon presumably needed line of sight to the caster, an unused reaction, and had to be within 60 feet.

BUT the DM hadn't used the spell before. So the players don't realize Counterspell is a thing baddies do. I dunno.

In my current campaign, my caster is really engaged. He watches for enemy casters to use their reactions. He tracks line of sight and distance. But he's been Counterspelled often enough to know it's necessary.

(This is my table, I don't expect others to enjoy this style of play, but as a DM, my final battles will not use mechanics the players aren't familiar with)

It also seems kind of spiteful? Revivify seems like an odd thing to counterspell, much better to stop a big damage spell or heal. Revivify only revives with 1 hp, so a strong breeze would put the barbarian back down. The party would have to commit multiple actions to healing him up while he runs for his life.

Instead, one of the players basically has to sit out for the finale of the campaign. Assuming the party succeeds, he can’t conclude the story with everyone else and the player just zones out during the epilogue.

I think a good twist mechanic is one you can react and adapt to. If the dragon countered a random damage spell, the players are introduced to the twist in a less consequential way and can now adjust their strategy to the new information. Here, they are punished for something they didn’t even know could happen (unless they have meta knowledge from other campaigns, which you should never assume) and can’t do anything except shrug and accept that their friend is dead.
my final battles will not use mechanics the players aren't familiar with

This seems kinda boring.

I pretty much expect a big bad to pull some off the wall crap I'm not expecting, otherwise what makes them big & bad?

The morale shattering of getting a revival countered would make the party come up with something new, instead of trying their same formula over & over. Remember the magic mcguffin they've been saving for a 'situation'? This is exactly the time to break out the one-shot items!
- There's still going to be all the standard lair actions, legendary actions, etc.
- I think the best is when you're exposed to mechanics one at a time, acolytes with one aspect of the BBEG's power, some creature with another, a foot soldier with a surprisingly formidable weapon, etc. Then, the finale combines them all. That way the experience is novel and challenging, but the players aren't blindsided when it matters.
Yeah but not everyone playing D&D knows all of the rules and spells and everything else, in fact, it's often more fun when players have less knowledge and get to experience things first hand and have a degree of mystery to them.
- Hard agree. My statement applies to my players at my table. I know them and they know the type of combat I run.
  
  5e isn't very crunchy. I think I would enjoy a more technical system that has more interaction between mechanics, but this is near the top-end of my table's level of engagement. And we're happy here.
As a player I'm constantly trying to derail the game and take the story in an unforseen direction. As someone that's looking for surprises and not trying to stay on the rails, I totally appreciate and applaud the idea of using mechanics I'm not familiar with. If I were that barbarian, I'd be very excited to have died like that.
- I totally appreciate and applaud the idea of using mechanics I'm not familiar with. If I were that barbarian, I'd be very excited to have died like that.
  
  As a DM, I would enjoy that. But since I put PCs in scenarios where there's a high likelihood of failure, I don't want them to feel like the physics of the campaign has suddenly changed during the last battle.
  
  I've tossed out new mechanics (doom trackers, Blades in the Dark style heists) mid campaign, but I'd prefer not to do that when we're coming to a conclusion.