Entity Framework support for Windows Store apps
System.Data.dll -- which is fundamental to EF -- is not available inside Windows Store Apps. Thus development of Windows Store apps is two steps back of where WPF and Silverlight development have been.
EF7 has initial support for Windows Universal Apps.
Dennis Apter commented
Having EF support for Windows Store apps would be HUGE for me as an app developer. I have several apps that require a complex (although not terribly large) data model. For example, I have one app that lets you create stop-frame movies. There are lots of bits of information about the frames, the animations, the sources of video and audio content, etc. that I need to keep track of. I would love to be able to read and store that data in small chunks as needed. Without a built-in database I'm stuck with reading it all from a huge flat-file and trying to juggle it in memory. SQL-Lite has been a major pain to work with from C#. I really wish I had access to the EF that I know and love.
The ability to write LINQ queries against a local relational database with full Entity Framework support will help us to reuse large amount of code for applications that run the same bussiness logic on desktop and on mobile devices.
The ability to write LINQ queries on Windows Phone against OData sources is sorely needed! Also the ability to write LINQ queries against a local relational database with full Entity Framework support, custom EF providers, etc.