I would disagree with both of the two posts above. You can't just make a big, two deck, four engined A320 and expect it to work.
Do you really think they would have had it over ten years in development if they were just making it a little bit bigger? Seriously, wake up and smell the sausages, Boeing fan-boys.
Obviously not, it's not a big A320, its an A340 with another A340 fuselage stuck to the roof.
But in seriousness while there is no one thing that makes in unique it is a whole lot of work to have actually got it up in the air. If it was easy to do I'm sure Boeing would have just extended the top deck of the 747 when Airbus originally announced A3XX project.
Bear in mind this is a massive aircraft that can use all the airport facilities, taxiways and aprons of most big airports, still fit in all the size limits and them fly several thousand miles more efficiently than a family car.
Why yes, I am an Airbus fanboy!