Inception is a perfect example of taking an old genre and updating it, as it is essentially a heist movie with the usually story of the "last" heist with the promise of freedom. What Inception did right was that it never went the conventional route like many other heist movies tend to do and the sci-fi elements of the movie keep the movie fresh throughout its run time. The casting was also inspired and the fact that Christopher Nolan is given free reign with pretty much anything that he wants these days added to the quality of the movie. People, when they first saw Inception in theaters they felt like it was something that they had never seen before and they are not wrong in anyway. I can't remember a heist movie based around stealing information while in a dreamworld but there were still plot devices used from other movies that led to the formation of the story.
A completely original movie is not a true possibility at this time in Hollywood because most ideas or the foundations of those ideas would have been used in someway over the 100+ years of the industry. I would like to see someone argue to me how any modern zombie movie/series isn't in some way influenced or based on the original Night of the Living Dead or Dawn of the Dead. Shaun of the Dead is one of my favorite movies of all time and it is clearly influenced by the classic mentioned in the previous sentence. It is a great original movie on its own merit and it borrows masterfully from it forefathers and there is nothing wrong with that because it takes the foundation from them and then creates an entirely new story on top of them.
Unfortunately, for every Inception and Shaun of the Dead there is a piece of shit like James Cameron's Avatar. The movie blatantly rips off story conventions and ideas from other movies and source material. Dances with Wolves, John Carter(THE BOOK), Fern Gully, and Tarzan are a few of the movies that it borrows directly from. Even the space marines are pretty much cookie cutter versions of the ones that appeared in James Cameron's Aliens. The idea of transferred consciousness comes from John Carter, the destruction of the forest for materials is straight out of Fern Gully and most of the relationship plot is ripped from Dances with Wolves. The movie has absolutely no heart or emotion to it and that contributes to the fact that it seems to be completely unoriginal in any way. It is just a bland big budget Hollywood production. It kills me a little bit inside that a director of James Cameron's quality will be make sequels to this piece of shit movie until the day that he dies.
There is a fine line that major studios must walk when they say that they want to release completely original content because it may come back to bite them in the ass. Tim and Eric's Billion Dollar Movie might be a completely original movie but it still sucks, because sometimes if you try and make something too original it comes out really weird and awkward. If you are able to find someone who is willing to take a chance and create an original story within an established sub-genre then you may strike gold such as Joe Wright's Hanna or Joe Cornish's Attack the Block. Both are personal favorites of mine and they just breakdown certain genres of movies and recreate them in magnificent ways. For all the "film aficionados" out there, I challenge you to sit through Salo: 120 Days of Sodom and still praise it for its originality and its artistic integrity. I would also challenge fans of James Cameron's Avatar to convince me that he did not ripoff several movies to make that movie.