The noise that greeted the news that Xbox One would now be backwards compatible with Xbox 360 titles said everything you needed to know about how big a deal that was.
This wasn't on the E3 bingo cards. This hadn't leaked out via the media - it wasn't even part of the conversation.
Yet gamers wanted it, so much so that it became the most requested feature on Microsoft's feedback website. And the roar from the crowd at The Galen Center in LA was all the evidence you need that this is a true killer app for Microsoft.
And it could prove to be the biggest weapon in Microsoft's arsenal in terms of winning over a reluctant Xbox 360 user base.
In the UK, there are over 9m Xbox 360 owners. If you consider that the install base for Xbox One and PS4 is just 3.7m, that shows the scale of opportunity Microsoft and Sony still have in the market.
Sony is resurgent. Yes, it's upgrading its own PS3 fans to PS4 successfully, but it has also been doing a fantastic job of converting Xbox 360 owners to PS4, too. Microsoft can't hope to win these users back - not yet anyway - its best hope is to convince those 360 fans who have not upgraded (all 5m of them) to join Xbox One. And backwards compatibility is a compelling argument. Especially as gamers can bring their games to Xbox One without having to pay an extra fee - a move that is in direct contrast to Nintendo and PlayStation's backward compatibility attempts.
Beyond this, a digital backwards compatibility solution is something that publishers can utilise to boost pre-orders. Bethesda is offering those who buy Fallout 4 a free copy of Fallout 3, while Ubisoft is giving away Rainbow Six: Vegas and Vegas 2 for buyers of the upcoming Rainbow Six: Siege. It's a low-cost way of encouraging gamers to get their advance orders, and could just as easily work for games like Mirror's Edge, Dark Souls III and pretty much any third party release on the platform.
It could also do wonders for pre-owned sales of Xbox 360 games, something I'm sure GAME, GameStop and the like will be keen to capitalise on.
There are still legitmate questions about Xbox One, such as how it can even possibly dream of taking back some of the European markets it has ceded so easily to Sony.
But as E3s go, this is the closest the company has come yet to shaking off that dreadful Xbox One reveal two years ago.