One of the biggest things that I've been excited about was the fact that WP 8.1 is supposed to actually have true device backup. The first Build session that I watched that touched on backup was The New Windows Phone Application Model | Build 2014 | Channel 9 which gave a small summary of the Data Backup and Roaming functionality.

Basically, what they talked about during this slide is simple. There are two options for carrying data between devices. The first is Roaming, where every application has access to 100KB of OneDrive storage. This storage does not count against the user's quota and is designed for stuff that's always updating and transferring between Windows devices (whether those are phones or PCs/tablets).
The second is that each app has the ability to backup its main data storage to OneDrive. This does count against the user's OneDrive quota, but is designed to be a more complete backup of all application data. It's backed up daily, and you have the option to restore to it whenever you set up a device.
So far, so good. There was then a more in-depth talk (which admittedly, I haven't seen yet - it will be posted in its entirety in about a day or two) today. I saw some slides from it that gave a little more information (on Twitter, courtesy of Justin Angel).

It wasn't until this next slide that I saw a problem, which is exactly what I was concerned about in the first place:

First of all, developers can opt out of having their data backed up. I have no idea why any developer would choose to do that, but it could be a bad experience for consumers, so I hope that none of them do.
Secondly, and far worse, is that for all current apps (Silverlight 8.0) and most games (XNA), the default setting is that data will NOT be backed up. Maybe for developers there would be a valid reason (and again I haven?t seen the talk that this slide is from), but for end-users this is absolutely terrible. It means that pretty much any app you have installed on your phone right now will need an update before its content will be backed up. So for all those old apps that aren?t updated anymore? You?re out of luck entirely. And even the stuff that is still actively worked on won?t back up its data until the developer pushes out an update either. So if you buy a new Lumia 930 to upgrade to from your 8.1 dev preview 920? Sure, you'll get new apps that do retain your data, but for most of what you're already using, you're starting over from scratch again. :angry:

Basically, what they talked about during this slide is simple. There are two options for carrying data between devices. The first is Roaming, where every application has access to 100KB of OneDrive storage. This storage does not count against the user's quota and is designed for stuff that's always updating and transferring between Windows devices (whether those are phones or PCs/tablets).
The second is that each app has the ability to backup its main data storage to OneDrive. This does count against the user's OneDrive quota, but is designed to be a more complete backup of all application data. It's backed up daily, and you have the option to restore to it whenever you set up a device.
So far, so good. There was then a more in-depth talk (which admittedly, I haven't seen yet - it will be posted in its entirety in about a day or two) today. I saw some slides from it that gave a little more information (on Twitter, courtesy of Justin Angel).

It wasn't until this next slide that I saw a problem, which is exactly what I was concerned about in the first place:

First of all, developers can opt out of having their data backed up. I have no idea why any developer would choose to do that, but it could be a bad experience for consumers, so I hope that none of them do.
Secondly, and far worse, is that for all current apps (Silverlight 8.0) and most games (XNA), the default setting is that data will NOT be backed up. Maybe for developers there would be a valid reason (and again I haven?t seen the talk that this slide is from), but for end-users this is absolutely terrible. It means that pretty much any app you have installed on your phone right now will need an update before its content will be backed up. So for all those old apps that aren?t updated anymore? You?re out of luck entirely. And even the stuff that is still actively worked on won?t back up its data until the developer pushes out an update either. So if you buy a new Lumia 930 to upgrade to from your 8.1 dev preview 920? Sure, you'll get new apps that do retain your data, but for most of what you're already using, you're starting over from scratch again. :angry: