Update #2: With all that power is it any surprise that Xbox One enables friends lists of up to 1000? "1000 friends is indeed confirmed," wrote the verified Xbox Support Twitter account, spotted by Polygon.
Update: It sounds like new Achievements can be added on the fly - the whole system is less rigid.
From Xbox.com: "Xbox One's all new achievements system has richer detail and spans across your games and experiences, which means achievements are no longer confined to a single game.
"And you can earn achievements in more ways since new ones can be added dynamically at any time. Our new achievements portal not only keeps track of what you earned, but how you earned it, so it's more personal than ever. It even lets you compare your achievements with your friends. Share your legacy and achieve greatness with Xbox One."
Original story: Good news: Gamerscores carry over from Xbox 360 to Xbox One, Microsoft has now confirmed. Your Gold subscription will carry over, too.
"Yes," Microsoft answered in an accompanying Q&A (via NeoGAF), "your current Xbox Live Gamertag will stay with you on Xbox One if you choose to keep it, and your hard-earned Gamerscore and Achievements will indeed carry over from Xbox 360."
Xbox One boasts a new generation of Xbox Live. The most radical change is basing lots of stuff in the cloud, where segments of games can be installed so they fire-up quicker and update in the background, just like on PS4. There's an auto-resume feature for games when you switch applications - either to check something online or do whatever - as well.
"You can save and store your personalized profile, games and entertainment in the cloud to access them any time, from any Xbox One console," Microsoft promised.
Xbox Live's Marc Whitten added: "Your content is available and it's stored in the clouds, so you can access your movies, your music, your games, your saves any time anywhere."
Powering all this are some 300,000 servers worldwide, Whitten said. To put that into perspective, Xbox 360's Xbox Live is powered by some 15,000 servers worldwide today. When the 360 launched, that number was 3000, and when Xbox Live launched on Xbox 1, that number was only 500.
"Game developers can take advantage of our worldwide multi-data infrastructure to drive direct game computation," said Whitten. "This means bigger matches with more players. It means living and persistent worlds."
New features of Xbox Live on Xbox One include a new matchmaking system called Smart Match, which estimates wait times and finds matches while you're doing other things, so you don't have to wait.
Achievements have been expanded to record videos of your "epic moments" - presumably those moments when you earn Achievements. There's talk of being rewarded in "new ways" but no detail on how.
Recorded video as a whole plays a much larger role on Xbox One, just as it does on PS4. "A dedicated Game DVR captures and accesses your magic moments, all saved to the cloud," apparently, and there are on-board editing and sharing tools.
Xbox SmartGlass - the tablet/second-screen application - is a fundamental part of Xbox One, having been tested out already on Xbox 360. "Now more devices can connect at one time for multiplayer and shared entertainment."
On top of that there's Skype integration - you can split-screen chat with people while doing other things - and there's the What's Trending? feature to show what's hot in games and entertainment.