The problem, as I see it, is unfocused storytelling.
It starts with a good screenplay that is focused, concise, and well paced. Shoot more than you need to tell the story, then edit out everything that's not 100% necessary. Be lean and purposeful in your story. A great screenplay will service everything in the story with great economy. Great scenes will serve two or more purposes, develop the story, and - when they're really great - make you lose all sense of time.
Many films are full of sprawling superfluous plots that don't amount to much. I feel this most during certain blockbuster films that are packed to the gills with spectacular set pieces, have three separate "emotional" climaxes, and somehow fail to stick the landing. When these elements are poorly balanced or overcooked they feel like a slog.