The word I would use to describe The Monuments Men is disappointing. It's an interesting premise based on the true story of a small group of art enthusiasts who are sent by the U.S. Army to find, protect, and return European works of art that have been stolen by the Nazi's in World War 2. While it is a story that you probably have never heard and was a pretty important mission, George Clooney as director just didn't deliver in telling a compelling story.
About halfway through The Monuments Men, I got the impression that Clooney couldn't decide what he wanted the movie to be. It was part comedy, part drama, and certainly part War movie (although light on the way plot) but it wasn't enough of either to be anything at all. He stops short of making the audience laugh, stops short of making them cry, and stops short of making them feel too much concern over the character. Furthermore, the third act had a promise of a climax that didn't deliver. There was no real tension in this movie. There was no excitement and there will be no affection for this movie from the audience.
Clooney did manage to assemble a good cast, but they were wasted in material that could decide what it wanted to become. It probably doesn't deserve a statement this harsh but right now, my overall opinion is that The Monuments Men is a monumental failure.