It's prevented from achieving greatness by a knee-jerk reflex on the part of writer/director J.J. Abrams and his co-writers Lawrence Kasdan and Michael Arndt to echo Episode IV of this saga at every available opportunity. This movie is executed well enough, in terms of both acting and cinematography, that it would be great if its script was a little less derivative. However, it is still better than the prequels and not nearly as stupid as Prometheus. In fact, children and people who haven't seen the "original" Star Wars movies (Episodes IV-VI) will love it, so feel free to introduce people to the Star Wars franchise with this movie. Just don't expect to be as overjoyed as they are.
Now, on to the detailed reasoning behind that assessment....
I only saw one of the prequel films (Episodes I-III for the Lucas nomenclature purists) in the theater, and I was late enough that I walked into Revenge of the Sith in the middle of the first scene. So it had been over thirty years since I had seen the opening title sequence of a Star Wars movie in the theater. I admit there were some chills when that card saying "A long time ago, in a galaxy far, far away..." came up, followed by the main title and the John Williams music.
And the echoes continued. The first shot after the traditional opening crawl was of a Star Destroyer filling the screen. Okay, this time the image was not followed by rebels scrambling to defend their blockade runner. Instead, we had shots of stormtroopers scrambling into transports for the attack. So, basically a mirror image of the opening of Episode IV. Then we get a confrontation between a villain dressed in black and an aristocratic person who gives them guff, and an important piece of information entrusted with a droid. At this point, I was wondering if this was going to be a beat-by-beat restaging of Episode IV with minor deviations.
Fortunately, The Force Awakens turned out not to be quite that derivative, but there are repeated callbacks. For example, a shot of the Millennium Falcon leaving a large spaceship's docking bay under fire is composed almost exactly like the image of the Falcon leaving Mos Eisley in Episode IV. Then there's the plot element of a chase across a desert planet in search of a droid.
Fortunately, there are enough original elements to keep things interesting, like the character of Finn, a stormtrooper who rebels against his conditioning when faced with the brutal reality of combat and the First Order's indiscriminate shooting of civilians afterward. His determination to get away from the First Order but unwillingness to directly confront it echoes a couple of different character arcs from Episode IV, especially when he interacts with other characters, but it feels genuine and not like a stale repetition.
Two of the most obvious echoes of Episode IV are the Starkiller base and the death of Han Solo at the hands of Kylo Ren. The first is a muddled attempt to make a bigger, badder Death Star. The second is an echo of one of Episode IV's most iconic scenes and is too obviously foreshadowed for viewers of Episode IV who are paying attention.
The Starkiller base is the worst-conceived, worst executed idea in the whole movie. The entire sequence in which it "destroys the Republic" is bizarre. We're never told exactly which planets it destroyed or why everybody on the planet where the heroes are can see the bolts travelling through space and figure out exactly which planets are being destroyed.
Then there's the whole technical explanation of how the Starkiller base works. The act of charging the thing's batteries creates an apocalypse in and of itself. Any organization other than the First Order would have just taken it around the galaxy draining stars of their energy. The new would-be Empire, on the other hand, decides this is not enough and figures out how to use that stellar energy to destroy planets from across the galaxy. The logic is never really explained.
Finally, there is the death of Han Solo, which closely mirrors the death of Obi-Wan in Episode IV. In both scenes, the hero (Luke in Episode IV, Rey in Episode VII) sees his or her mentor figure (Obi-Wan in Episode IV, Han Solo in Episode VII) getting killed by the main villain. Because of the script's careful foreshadowing, I could predict the end of this scene from its beginning. The resonances with Episode IV were just too carefully constructed and pointed out. That killed the impact of what was supposed to be the most shocking plot twist in this movie. It's a shame, too, because in some ways the scene itself was better than its inspiration in Episode IV. There are higher stakes for both audience and characters. While Obi-Wan was somebody we had just met in Episode IV (for those of us who didn't watch Episodes I-III first), Han Solo is somebody we have a long history with. While Obi-Wan is just trying to get past the apprentice who disappointed him and onto the Millennium Falcon, Han is trying to redeem his wayward son so he can face his estranged wife without feeling like he failed his whole family. This scene should have been an unqualified triumph. Instead, it falls flat because of its obvious setup.
Still, Episode VII is a solidly executed film that sets the franchise back on solid footing. I just wish it wasn't using its predecessors as a crutch.