I think fundamentally an adversarial DM is one who views player losses as some sort of victory for themself.
I recognize that my EXTREMELY strong feelings on this regard are unusual, so I don't try to harp on it too much. I'm a very strong believer in the ideas of GM sovereignty and player sovereignty. Building the characters that you want to is one of the hard and fast rules of player sovereignty that I have, and if I GM is going to "punish" them by not bringing a game that works for the players that he has, as opposed to some platonic ideal of a party, then to me that's adversarial and passive-aggressive.I disagree. It's not adversarial to have a world that exists independent of the PCs. That's strong worldbuilding. If it's a party of all squishy wizards that doesn't mean they'll never face a well-armed and armored melee combatant. You don't give out extra healing potions because no one wants to play a cleric. To me, that's a player-side problem. They can generally be assumed to know what to expect, if they choose not to prepare for that, it's on them. They can easily pick up a hireling or two to cover their deficiencies. But if they choose not to...that's on them. A world that's always shifting to perfectly suit whatever the PCs have in tow is way, way too video gamey for my tastes.
5e difficulty is entirely down to the DM and the table. Difficulty is within the control of both players and DM. If players want to play on hard mode they just don’t select healing word. All of a sudden 5e is hard as any edition since 2nd.Sure. I think that might be a player empowerment thing. 5E has shifted so drastically to the non-challenging side of assuming the players must always win that it's pointless and boring unless the difficulty is cranked way up. The game went from "zero to hero" under TSR to "superhero to superer superhero" under WotC. The players just assume they're badasses and will easily win everything all the time. That's dull. Still, no reason to mock them and cackle. Maybe play an older edition or talk about house rules to tone the characters down.
An important distinction because I expect my GM to provide opposition, I dont expect them to want to defeat me in RPG terms.To clarify do we mean "adversary" as "providing opposition" or "adversary" as "enemy"?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.