Here's what's new.
More Reading View
The reading view in Office Online has always been very basic--little more than an annoying extra step to pass through before editing your document, really. With the new updates, Microsoft is making reading view a little more useful, first by switching the options interface to the right side of the screen instead of the left.
The option to print a document is now front and center instead of buried under the File menu. There is also a new generic menu hiding a few more features, such as a translate option, the ability to view past versions, and embed the document in a web page.
More file management
Previously, when you clicked File > Save As in Word Online you were met with two options: download a copy of the document to your desktop, or download the document as a PDF. With the upcoming improvements, Microsoft will also add the ability to save another copy of the file to OneDrive and introduce an option to rename a file.
Add to OneDrive
Making it easier to edit files that aren't your own, Microsoft will now let you add a copy of a file to your OneDrive if it's owned by somebody else. Say, for example, a colleague shares a memo with you over OneDrive but with "view only" permissions. Clicking the new Add to OneDrive button will let you save your own copy of the document that you are free to edit.
Other improvements
Those are the big changes, but Microsoft is also rolling out a few other tweaks, such as an improved start screen that more closely mimics what you see on the desktop with Word 2013. Microsoft has also improved the "Tell Me" natural language help feature, and added word counts when you highlight a section of text. Office 365 subscribers with Android tablets will now see an Open in Word button just like desktop users do, allowing them to open documents from Office Online in the native apps on their device.
The new improvements are rolling out now. You may see some of them in your account already with others to follow soon.