Must Have List
1. Tactical Combat: If you can mindlessly repeat the same attack pattern and win it doesn't require tactics. BG2 had a great experience where the priests of the Blind Beholder kept whupping my party. I searched through the character's spell-lists for some game changer and found the druid's bugswarm would silence multiple enemies. That one change flipped an overwhelming opponent into a vulnerable opponent.
2.Good quest design flow with multiple ways to enter, end, and progress through a quest.
3. Reactivity. Even small touches of polish like Deus Ex having JC Denton's boss scold him if you run into the women's bathroom made the world feel alive.
4. Leverage Character Stats outside combat/rolls. The Sith Lords had a lot of speech content options which were unlocked based on the intelligence stat; Bloodlines had dialogue exclusive to Malkavians which made for some of the most memorable scenes (talking to the tv, talking to the stop sign), Fallout 2's low-intelligence playthrough was funny, while the character was too dumb to understand most quests, he understood the meaning of a crude brahmin drawing on the town's bulletin board.
5. Many playthrough paths > longer main story playtime. Data from games like Days Gone's main story completion show less than half of players beating the game. ME2's official release on the same data years-out was also 4x% of players beat the game. Better to have a shorter single-playthrough duration and more character-build-derived content to reward repeated playthroughs. That way casual players see the whole main story and gamers get to see different content depending on their choices, like in The Witcher 2 where the 2nd act of the game is completely different based on who you leave the starting-area with.

Do Not Want
Preached to from the Frankfurt School.