I don't really have one. I just know one when I see one. I was never into the debating often seen on rpg forums of "is this a rpg?" or, "is that a rpg?", to be honest.
I can tell you what I like, as did other comments.
I like fantasy worlds, specifically classical fantasy. Dragons, wizards, orcs, etc. with a mysterious world and setting with rare weapons of magical power and other treasure to be found. Not too much treasure though! This is a problem with even modern shining examples of the genre, games as fine as Divinity original sin - they are scared to not give the player enough treasure items in the form of gear, so they overload it. And that is a mistake. You should find mostly junk weapons, most of the time. Or, perhaps just a little bit better than junk, on occasion. A powerful magical weapon/armor should be rare to find. That makes it much more special and significant for the player, upon discovering and being rewarded with these items, rather than showering him with tons of magical goodies, constantly. (unless I suppose if its an action rpg - that is different)
The ideal is isometric "top down" view. This is because the best rpg is a multi-party type of game. A party of at least 4 characters. This is to allow for the classic strategy of one wizard, one thief, one fighter, and one cleric. It doesn't get much more basic than that, folks. But of course if 5 or 6 characters in the party? - even better.
Finally, a D&D-like system of character progression. This makes it fun to level up and get more powerful. To be more basic, there must be an ability for and a focus on character growth. Perhaps the most important factor of an rpg, in my humble opinion. Interest in the game would diminish quickly without a hook like this. If not, then we can at least say the game doesn't really resemble an rpg, because this is a vital element of the rpg.