Pacing is. I recently just re-watched Rocky, Pulp Fiction and Godfather - which have been my favorite movies for past 20 years only to end up a bit disappointed. On the other hand I've watched a fair share movies that are rated 5/10 on imdb that I find much more enjoyable and it took me a while to figure it out..
It's pacing. That's how you keep watchers interested.
Or maybe I have a lack of attention span.