To begin with, you now have the choice of signing in with your Xbox Live (really, Microsoft) account -- and it'll eventually be mandatory. This could create hassles for streamers who'd prefer to keep their Beam and Microsoft accounts separate, but the team is counting on Xbox Live's features winning you over in the long run. It'll help out with built-in Beam streaming on Windows 10 and the Xbox One down the road, of course, but it'll reportedly help out with two-factor authentication, moderation and other aspects of the service.
On top of that, you should also see much faster site loading (up to 5 times faster, Beam claims) and visuals, a better viewing experience on the mobile web, and a slicker chat experience that includes emote auto-completion. You'll have to wait until later in the year for Xbox One support and new mobile apps, but serious streamers who use XSplit will be glad to hear that a beta release now plays nicely with Beam's low-delay streaming (FTL) as well, just as OBS has for a while.
We'd note that things appear a bit rough at the outset. Don't be surprised if you run into a broken feature or two. However, you do have the option of rolling back to the previous interface if you dive into your account settings.
Will the overhaul convince Twitch die-hards to switch over by itself? Probably not -- the temptation to stick with the community you know can be quite strong. It shows that Beam and Microsoft are committed to making their service a force to be reckoned with, however, and it's laying some important groundwork for Microsoft's big gaming initiatives this year.