Backwards compatibility. It's the number one reason why people I speak to say that they won't be upgrading to a next gen console. At E3 Microsoft just gave Xbox 360 owners an upgrade path that removes that particular complaint.
The change of architecture between the current consoles and last gen boxes was always going to prevent true backwards compatibility, however Microsoft have created a workaround using an Xbox 360 emulator on the Xbox One, and a process for packaging games as downloadable content. Insert your 360 game disk into your new console, wait for the repackaged game to download and then its playable, requiring only that the original disk is in the drive to play.
At launch - before the holiday season according to Microsoft - there will be around 100 games re-packaged, with hundreds more to follow so there's a pretty good chance that your favourites will be in there.
For Sony it's a blow, as it makes it much harder to convert Xbox 360 owners to a PS4, given that there is now a clear upgrade path on the Microsoft platform.
What with this and the news of a new professional level and highly customisable controller; new games and the availability of Xbox One to Windows 10 game streaminf its been a pretty good E3 for Microsoft. Be interesting to see how Sony responds.