I've finished the last Harry Potter novel and found it good. While I can nitpick the writing and a couple of plot holes (how did the Sword of Griffindor get from the grasping hands of a goblin back into the Sorting Hat?), it's a well-done close to a masterful work of imagination.
One of the things that makes J.K. Rowling's novels work is that they describe a universe with its own rules - its own physics, if you will. People can develop new spells and invent new magical gadgets, such as Dumbledore's deluminator, but they can't break some essential rules such as magic's inability to bring back the dead. The capabilities and limitations of magic in Book 7 are the same as in Book 1. The same can be said of J.R.R. Tolkein's magnificent Rings saga.
Compare that to George Lucas' Star Wars saga. Originally, Lucas set a rip-roaring adventure tale in a space world that looked lived in and was basically consistent under its own rules (allowance of hyper-light travel, etc.) By the second trilogy, though, the rules kept shifting. The Force, an innate capability that "binds the universe together" and could be developed by practice, suddenly became a function of genetics (a person's midichlorian count). The laws of space physics, which had been the physics of our universe with a few settled exceptions, suddenly went out the window - literally - as spaceships pulled up alongside each other and blasted with advanced versions of 20th century cannon - though with recoil having no effect on the motion of the spaceships.
The point? Our universe, and the science that describes it, operates by a set of physical laws that can't be broken (they can be suspended if you believe that God sometimes takes a direct hand in human affairs, but that's beside the point of this discussion). We don't know all the nuances of these laws, and, as we understand them better, we may create radical new technologies (e.g., sending information via the "spooky" entanglement phenomenon, or, more distantly, making use of wormholes for travel in some form). But the laws are consistent in the real world. We can accept variations or even entirely new physics in fantasy books and films, but those variations work when they are part of a consistent author-created universe. Establishing rules and then abandoning them is highly unsatisfying for the reader/viewer (and, George Lucas, beneath your considerable talent to rely on).