Still haven't seen a coherent definition of the I-word, but....
....I have never forgotten I'm playing a game, as far as I can recall. But that's fine, because my goal is to play a game. :shrug:
I think immersion is the result of how much you like the game, on several levels. A game you don't like is because the graphics are bad or just not your style, bad mechanics, bad controller layout, [censored] audio (bad acting, weak effects etc) and so on, being a combination of these or just one of them.
It's not the game that is immersive, but how you experience it.
Played many games in my life that have done a lot of things right, but none are them are immersive. It's always just a game, and it always makes sure to notify me of that.