In a word: development.
George Lucas had the vision and ability to unfold scenes that were slices of an epic story, not just connected pieces of one story.
In IV, consider the first scenes with Luke and his uncle and aunt. There are conversations about washing droids, cleaning up, going to town, fixing water harvesters. They talk about dreams and hopes for the future. It's not only the content; it's the pacing ... not only the pacing, it's the flow. The first Luke scenes don't seem like an integral part of one main story - though they are. They seem like the middle of another story and a typical slice of life during that story. Until the first visions of Leia in the hologram there is no feeling of being part of a "plot". From the hologram until C-3PO mentions that R2 ran away, again there is no feeling of rushing forward in a plot. When exciting things happen, it feels like worlds of different stories are intersecting and getting caught up in each other.
You get the same feeling watching Luke fight the battle ball - it's a slice of a longer time period, making you feel that much time has passed, and Luke has experienced similar events, and many more will come. When Leia is interrogated, same thing. In VII, the interrogation scene we saw was the only one that happened. In VII, every scene felt like it existed only to get to the end of the scene - okay, got it? Let's move on to the next. The process of getting there was clean and executed quickly and antiseptically. The movie needed to tell you this to move the plot forward.
VII gives us just one scene that feels like a slice of time - the scavenge scene and the cleaning of what was scavenged. It lasts about 2 minutes. Even the meeting of Rey and Finn is a sequence of bursts: a fight, then a challenge, then seconds later we're on the run. Then 40 seconds of trying to fix the ship, then a capture. Then a brief exposition by Han, and then another challenge/fight. After the scavenge scene, VII NEVER feels epic. It always feels like a rush to finish a story that is already completely scripted. Even the one scene that should have been filled with wonder - Maz and Rey and the lightsaber - was a rushed scene to get to the point, and then to the end, and then we run off to the next action sequence.
Remember V, with Luke training on Degobah? There was no rush to get to the end of it; it felt like slices of weeks of training. Remember VI, with the walks through the woods? Even in I, II, and III there were (a few) scenes of patience: they may have been terribly acted and poorly scripted, but they were scenes that gave you the sense of epic, long-haul, world-building, not just plot-driving.
That's the first reason.
The second reason is one that others have mentioned: There is nothing new in this movie, other than a female protagonist. Every Star Wars had new ships, new worlds, new creatures, new weapons, new plots, new conflicts, new robots, new costumes, new discoveries, and so on and on. What was new in VII? Takodana looks a lot like the moon from IV, as does its cantina. The Starkiller world has snow, which could have been interesting, but we didn't get to see any of it. The rest was just everything we've already seen.
Again: it was an enjoyable sci-fi movie based on the Star Wars story. The acting was great, there were good lines, it was fun to see a tough, non-sexualized capable heroine. But there was no character development, no epic story, and nothing mystical, at least not until the very last frame. Maybe, maybe, the next movie will give us something more than a very good Star Wars version of a Marvel movie.
See here for my initial review.
The Real Reasons The Force Awakens Fails as a Star Wars Movie
In a word: development.