I recently got into XCOM a few months back, and my first comment on it to my friend was "this is a better roleplaying game than any roleplaying game".
It's hard to communicate with precision what I mean, but it's something like, the combination of clear systems that are immediately understandable, and the ability to make reasonable predictions about the effects of your choices, and the way every decision you make really does matter. In short, you genuinely have agency in the game in a way that I rarely see in video games (outside of, for example, strategy games where you're competing against one or more players, but I consider these to be a separate sort of experience).
Here's an example that will hopefully help illustrate :
In the very first mission, the objective was to break into a facility and perform a task like steal an object or blow something up (it's been a while and I forget the specifics, and this part isn't that important). Anyway long story short, after several skirmishes and achieving the objective, my squad was down to literally one guy. He just barely made it out to the extraction point while under fire from alien overwatch teams, and with more reinforcements arriving shortly.
In a lesser game, what I described above would sound like something that would happen in a cutscene. It was like something straight out of an action movie. But in this game, it was entirely the result of systems in the game that worked no differently during this situation than in any other, and the result of my decisions interacting with those systems. There wasn't a GM arbitrarily deciding to kill off players because "it would be cool and dramatic". They died as a result of my own decisions. There weren't overwatch teams positioned along the extraction route because the GM poofed them into existence to make the encounter more challenging or dramatic. They were there because the aliens who were already in the location actively decided to take up those positions.
No opponents ever arbitrarily received more health because they were dying too fast, and no BS happened to save my guys from dying. I was never granted new capabilities or weapons in violation of the rules because it would be more cool. Whatever the math and the enemy AI and the randomizer and my decisions determined, is what happened.
This is, basically, what I want from a roleplaying game, and it is what I aspire to when I run them. Things shouldn't happen because of or in service to a story. They should just happen, because of either the laws of physics, the system rules, or decision making. The story is this post. It's what we tell each other about what happened. It's not the act of playing itself.