I'll try to describe my feelings towards this movie using a technique called layering. The first level shall be a blatantly general statement followed by more levels drilling down to reach a final depth and opinion.
21 grams -- opinion in three levels or less:
Level 1: 'Lost in Translation' cinematography meets 'Adaptation' story line.
Level 2: Characters' truthful and morbid life-tale; of gain by the misfortune, anguish, and loss of others.
Level 3: A stark glimpse into the effects and affects of a single event in time; of the events that led up to and that followed, to come to one cohesive point, that changes the lives of all involved.