What is that one feature in adventure games which makes you automatically gravitate to the purchase button?

For me it's the inclusion of multiple endings. The more, the better.

I'm guessing you've played Titanic: Adventure Out of Time, which has 8 different outcomes. I've played it 3 times with 3 of the endings.

I'm drawn to first person games.

Slider puzzles!

Okay, not really. The only time I will automatically buy a new game, no questions asked, is if it’s by a dev that I love.

Interesting female protagonists in adventurer roles (unlike "So Blonde").
The graphics style (I like it "unique", hand drawn, stylized ... rather not too realistic).
For example I bought The Cat Lady and was amazed.

But there is never an "automatic purchase" for me.

There are other things that are more important to me than just the graphics, but I won't know if the game has them until I have played them.

I'm guessing you've played Titanic: Adventure Out of Time

No, I haven't played that game. Thank you for the suggestion, LadyKestrel.

which has 8 different outcomes

The Council is an adventure game that I consider to be absolutely fantastic. 14 Endings!

I'm drawn to first person games.

With good-faith intentions by the developers it creates a window to another dimension that is truly immersive. With bad-faith intentions it's a way to cut production costs by not having to draw and animate the protagonist.

Slider puzzles! 

I've never understood the disdain that most adventure game enthusiasts have for slider puzzles. I actually really enjoy them. They are just a relaxing filler puzzle for the purposes of pacing. They are quite a common occurrence in Nancy Drew games.

is if it’s by a dev that I love.

Jen, don't leave me hanging, what is one of your favourite developers?

Interesting female protagonists in adventurer roles (unlike "So Blonde").

Hexenwerk, you've now got me curious, where does Nancy Drew fall on this scale of yours.

But there is never an "automatic purchase" for me.

I wrote the question with just a dash of hyperbole. Listing a number 1 ideal feature is fine.

Nancy Drew, hm. I never played any of those games, so I don't know. Maybe they are not for me as first person games (you don't walk freely? I need to check that out - I have problems with orientation in certain player camera settings). For example What Remains of Edith Finch was fine for me. So maybe there is a Nancy Drew game I might play as well. I am curious about "What was found at Raven Hill", for example.

Kathy Rain, Technobabylon or Whispers of a Machine, Fran Bow, Edna & Harvey, Little Misfortune would be games that convinced me instantly. I know they are dark/mystery games, so that might be something I like as well. But what I liked more about Kathy Rain was the detective gameplay. So "detective mechanics" would be something to check as well for myself.

I actually don’t hate slider puzzles, but there was a time when it was de rigeur, it seemed, to put one in a game. They just seem tedious and unoriginal to me. I’m actually very good at them now. I learned by playing adventure games. 

My current favorite developer is Glitch Games, on iOS. I don’t know if any of their games are on PC or Mac.

I'm not fond of sliders, especially if I have to guess the picture, but I can work through them.

Of course, there could be an alternative:
   

A new point-and-click adventure game that doesn't mention "Monkey Island" or "classic point-and-click games" in their Steam description.

A new point-and-click adventure game that doesn't mention "Monkey Island" or "classic point-and-click games" in their Steam description.

Exactly! Don't ride on the coattails of other's people success with games like Monkey Island, instead just let the qualities of your adventure game stand on their own merit to become readily apparent by the player.