With all of the new technology that is being implemented with the Xbox One, the price you have to pay is that the console will not be backwards compatible with the previous generation’s Xbox 360.
It wasn’t announced during the ceremony but after the event in a quick Q&A with reporters Marc Whitten, Microsoft Xbox Live VP, stated “No there’s not,” when a reported from The Verge asked about backwards compatibility. This is due to the fact that the new Xbox One will host an x86 CPU architecture. That dramatic jump will not allow previous games to run properly.
This shouldn’t be too surprising, as the PS4 and many consoles in the past have suffered from the same problem Sony did mention that PS3 players could run games off the cloud, meaning in the future you could see your PS3 library renewed. Microsoft made no such promises.