The guide has dramatically changed, with Xbox engineering team member Richard Irving describing it as the "big focus for this preview release." While on the Home screen, it's now accessed by scrolling over to the left side of the screen. While in-game (or any other activity), double-tapping the Xbox button immediately brings it up without any loading time.
Either way, it's now a column along the left side of the screen--the new location for anything you choose to Snap. It offers access to things like your friends list, party, messages, notifications, and headset settings (such as volume, muting, and more). Notifications, in particular, have been overhauled, as they are now grouped together based on their type or the game/app they originated from. In other words, recording a bunch of game clips won't force you to scroll through a long list to reach, say, an achievement notification, as is currently the case.