well, i agree, tongoll, that character development is another important tool.... sometimes strong character development can overshadow and make up for a weaker or more derivative storyline... strong characters can be very compelling, and can definitely draw a viewer in. though, at times, a strong and compelling story can make up for slightly weaker characters.... but if you have strong character development, and a strong story, then you are on your way to a good film.
mood and atmosphere can still make or break the film... just like any other element can... each element/tool is important to the overall effect of the film. write a good story, write a good script, develop characters well, cast good actors for each roll, create the right mood and atmosphere, shoot in an interest and effective way that fits with your mood and story.
i don't think there is any one thing that is always responsible for making a movie compelling... it can be strong characters... or an interesting storyline... or maybe snappy dialogue in the script, or maybe an interesting mood or atmosphere.... as i said, all of these are tools that the filmmaker can use.... and each of these can ruin the film, if done badly, or help the film, if done well. the more care and attention that is given to each of these elements, and the better they are done and fit together, then chances are, the more compelling the final film will be.