The most recent issue of the Journal of Quantitative Analysis in Sports includes an article entitled "Study of the Technical and Tactical Variables Determining Set Win or Loss in Top-Level European Men’s Volleyball," by David Rodriguez-Ruiz and colleagues. The abstract is available here; full-text access requires a subscription, but guest privileges to view an article are available. Based on study of the 2009 European Men’s Volleyball Championship, the central finding appears to be that whereas "attacking [tended] to be the highest point-scoring technical action," the block took on greater importance the closer the match.
NOTE: Leo van Hal was kind enough to share his reaction to the aforementioned study, in the Comments section (link below). Leo also had a graph to accompany his comments, which doesn't work in the Comments section. I am thus reproducing his graph here: