Game Mechanics That Remind You It's a Game
That up there is The Division -- a game with utterly lifelike graphics, right up until the point you shoot someone and make numbers fly out of their head. And I know that's just a minor thing, but it makes my point: Games are in a weird tug-of-war between visuals that suck you into the universe and mechanics that yank you out of it. Little arbitrary rules that only make sense because it's a "game."
For example, like a lot of you, I like to fly into a rage at the slightest provocation if things don't go exactly my way. I therefore don't have a lot of tolerance for doing the same shit over and over with no progress, and as such, video game difficulty is something of a sore subject with me. My gaming time is very limited, and I want to progress to the next thing. But I'm fine with a game beating me, as long as A) it's fair and B) it makes some kind of logical sense.
For the record, forcing you to buy power-ups is neither of these things.