There's a standby piece of criticism – positive criticism, mind you – that I seem to hear being thrown around a lot in film school: I forgot I was watching a movie.
First of all: I know you don't mean it literally, but it sounds dumb. It makes me think of the fabled first-ever film audience who all ducked down and screamed when they saw a picture of a train heading toward them on the screen. Maybe I'm alone in this, but I'm hyper-aware of the fact that I'm consuming media – even when I'm in the act of consuming it.
Second of all: it's vague. Really, really vague. Try to think: what was it about you were watching that supposedly had this effect on you? It'll be more helpful for both you – after all, it's nice sometimes to know what's going on in your own head – and to whoever made the film, if they happen to be around. Was it that the dialogue was realistic? Say that. Was it that an actor gave an emotionally gripping performance? Say that. Was it just that the pictures onscreen were really, really pretty? Say that.
But don't just say that you forgot you were watching a movie. It doesn't work when describing what makes a film compelling. It certainly doesn't work when you apply to compliment to something that isn't film. "Wow. This beer is so good that I forgot that I was drinking it."
Okay. Actually, in a way, that one works. But for different reasons.