1. Setting and lore.
This is the single most important thing to me. The core of the game has to be built on solid ground. The game's over all setting has to make sense to me. Skyrim for example while not winning the most orginal story of the year awards, shines creativity when it comes to world design and lore. Elder Scrolls is such a vast and rich setting…
Fallout new vegas was an other game that really sucked me in. Many say that fallout 3 has more intresting gameworld to explore and that I do not disagree, but its a messy gameworld which at times makes little sense. Obsidian understood the core of fallout better and thus built a more memorable game.
Star wars, Divinity: Orginal sin, Wasteland, Mass effect, baldur's gate, Gothic, Arx Fatalis, Deus ex are also examples of strong settings.
2. Exploration
I wish to stray away from the beaten path and It always feels slightly dissapointing when a game doesn't let me do that. I can enjoy more linear games, but only if the game compensates it's linearity with good story or great combat mechanics for example.
3. Npc interaction. If characters are writen poorly, the plot of game rarely matters to me. If i don't care about the characters, why would it matter what happens to the world where they live?
4. Character creation. While Witcher, Deus ex and Gothic series prove that great rpgs are possible with preset characters, but its usually for a reason considered one of the core pilars of a rpg.
5. Story + choises & consequence. The dream of mine is ofcourse that a memorable story which is both dynamic and reactive to player's choises is built on top of those four pilars.