I said "something else". For me the overall impression is the most important reason to like a game. It is like with a good book, a good movie or a good piece of music: one aspect alone (like plot, quality of writing, setting etc.) doesn't make a good work of art.
A combination of setting, atmosphere, lore, story and exploration is the most interesting part for me. Mechanics, in particular the fighting mechanics, is like the salt in the soup: It isn't tasty without it but too much salt makes it bad too.
The "gamey" aspects of the mechanics (i.e. challenges to my skill in playing the game) are not important for me, if I want something like that I play chess or table tennis but not a computer game.
Edit:
I realized that the things said above are the reasons to like a game, not the reasons to buy a game. This is a difference since you don't necessarily know, whether the game has the properties you like. So my answer to the question should rather be: If I assume from reviews and experience reports, for example from people on RPGWatch, that a game might be as described above, I will buy it. Also in many cases, when I know the developer and expect my type of game from that experience.