Painfully average There are arguably 2 major issues this movie has: the CG and the script.
When the movie started, it opened with a gorgeous African view and I thought to myself, "Well, even if it is not going to be good, it will at least be pretty". Movies set in Africa automatically get extra points just because they are set in a country that is naturally beautiful and atmospheric. However, as the movie continued, almost everything except the actors was computer-generated, including things that did not need to be computer-generated, like trees, or a train. There is a scene where a man is walking his pet crocodile. They do not show the man's face so it would have been easy to hire a trainer with a real crocodile but even this croc was fake.
CG is always going to be an issue when you chose to make a film with animals as active characters, but another such film, The Jungle Book, at least made sure that everything looked impeccable. The same cannot be said for Tarzan, with its fake blurry steam train looking like it came out of an Xbox game. So much CG pulls you right out of the movie.
Poor CG however does not break this movie. The terrible script does. This problem can be roughly divided in 3 parts: the slow first half, the bad exposition and the poor dialogue. The first half of this film has a pacing problem, as it takes its sweet time with setting up the return of Tarzan to the jungle, suffering under the weight of the need to connect the current events to the original story of Tarzan through countless flashbacks, which brings us to the bad exposition. There is so much of it and it is so blunt. Every character's motivation is spelled out, and everything that is not talked about, gets shown in flashbacks. Can't find a clever way to explain what is happening? Just let the character describe it out loud. Film-making! And none of the dialogue is particularly good, nor does it fit well with the action, so all that extra talking really doesn't do this movie any favors. On top of that, there is a scene towards the end in which a character does something that makes no sense within the scope of this story, a final nail in the coffin for this flick.
Perhaps this film could have been saved with good acting. I only watched this movie because it promised Christoph Waltz, and his performance is fine, but there is a scene in which a Belgian officer played by Simon Russell Beale talks to the character of Christoph Waltz, and the performance given by Mr. Beale is considerably more captivating than that of Waltz, and Christoph Waltz is supposed to be the main villain, for crying out loud!
While this movie is not a complete wreck, it is painfully average, and in a world with so much other entertainment available, "average" might as well mean "useless".