First, of course, is Nolan's direction. There is some seriously enlightened direction going on here. It starts off a bit slow (as most of his movies do), but he shortly throws it into overdrive. Before long, the movie is roaring past you. It's exhilarating, simply put.
Nolan's writing is also top notch. The dialogue is refreshingly real, sharp, and coherent. In the beginning, viewers might find themselves a bit lost (don't worry, it's intentional), but Nolan slowly exposes INCEPTION's secrets. The plot direction carries this movie from being a simple heist movie (in the same vein as, say, OCEAN'S 11) to an emotional, thoughtful, intelligent, and utterly astounding thriller. If Nolan doesn't receive a directing or writing Oscar for this movie, then there is something seriously wrong the Academy.
Also worth mentioning are the visual effects and stunts. The VFX are seamless. Even in the film's most visually intense moments, it's not entirely evident that Nolan is using CGI. Of particular mention is a fight scene that takes place in a unstable-gravity hotel hallway. As the actors twist and writhe all over a room where gravity is in flux, the viewer is left dumbfounded by the surprising realism of the scene. There's no other way to say this: INCEPTION looks great.
Finally, we come to the acting. If anything could steal the spotlight from Nolan's directing and writing, it would certainly be the efforts put forth by the cast. Overall, the cast has impeccable chemistry. Everyone plays together so well. - Of course, Leonardo Dicaprio was solid. The role isn't much of a deviation for him, but he plays it well. His most emotionally intense moments hit hard and his character is developed very well. He deftly leads a relatively-unknown, but unbelievably-talented cast. There isn't that much "star power" in this movie, but the caliber of the acting is superb. - Marillon Cotillard was unbelievable. Her role is arguably the film's most versatile and she expertly glides through each aspect of her character. She's a very talented actress who works so well with Dicaprio. Their relationship is one of the emotional high-points of the movie and that is thanks entirely to each actors' powerful contributions to the heartbreaking reality of the relationship. - Ellen Page did a great job in her supporting role, bringing some of her signature wit into it. She has a very big on screen presence and she fits in surprisingly well with the mostly-male main cast. - Joseph Gordon Levitt and Tom Hardy stole the show, in my opinion. They each play two very well-crafted characters extremely well. Levitt plays a resourceful, no-nonsense point man that wonderfully counters Dicaprio's somewhat-unstable character. Hardy plays a charismatic thief that goes from devious trickster to full-blown action hero. These guys have some seriously impressive acting chops and it was truly a delight to watch their performances. - Cillian Murphy, like Dicaprio, is mostly sticking to a role that he knows, which is not a bad thing. Murphy is a talented actor, which is readily apparent in his last big scene in the movie (one of the movie's most emotional moments). - Ken Watanabe and Dileep Rao also add a lot to the overall chemistry of the ensemble. Their presence, though not as major as some of the other characters, definitely help solidify the ensemble's power.
INCEPTION will stand the test of time as one of the great American films. Nolan is a superb director and cunning writer who has assembled an elite group of actors to create a visually stunning, thrilling adventure. You'd be crazy to not want to go along for the ride.