Updated: with EF Core 1.1.
Here’s a brief summary of some of the features that were present in previous versions of Entity Framework (6.x) and were excluded (or are not yet implemented) in Entity Framework Core 1:
| Feature | Description | Reason/Workaround |
| Entity Type Configuration | The ability to load entity configuration from classes in the same assembly as the context (EntityTypeConfiguration) | Will be available in a future version. |
| Lazy loading | The ability to load entity relations after the root entity was loaded, automatically | Will be available in a future version. For now, we need to use eager loading (Include). |
| Explicit loading | The ability to load entity relations after the root entity was loaded, explicitly | Will be available in a future version. For now, we need to use eager loading (Include). https://github.com/aspnet/EntityFramework/issues/3797 Implemented in EF Core 1.1. |
| Support for Group By | Specifying GroupBy in a LINQ query | Currently it falls back silently to LINQ to Objects, meaning, brings everything from the database and groups in memory. Will be available in a future version. For now, use plain SQL. |
Support for user defined functions | Using static methods as UDFs | The ability to use .NET static methods as User Defined Functions (DbFunctionAttribute) is not supported now. Will be available in a future version. |
| DateTime Operations and common SQL functions | Doing DateTime operations and using common SQL functions in LINQ | The ability to do queries involving Date/Time operations (DbFunctions) and common SQL functions (SqlFunctions) in LINQ queries is not supported now. Will be available in a future version. |
| Complex Values | Support for properties of complex types (value objects) | The ability to use properties of complex types is not supported yet. Will be available in a future version. |
| Many to Many Collections | Many-to-many relations without a middle entity/table | Will be available in a future version. For now, we need a middle entity and table. |
| Table Splitting | Will be available in a future version. | |
| Table Per Type Inheritance Strategy | Will be available in a future version. | |
| Table Per Concrete Type Inheritance Strategy | Will be available in a future version. | |
| Mapping CUD with stored procedures | The ability to use stored procedures for doing inserts, updates and deletes seemlesly | Will be available in a future version. |
| Map database views | The ability to map views instead of tables | Will be available in a future version. |
| Spatial data types | The ability to query and use spatial data types | Will be available in a future version. |
| Custom conventions | The ability to add custom conventions | Will be available in a future version. |
| Populate non-model types from SQL | Turn the results of custom SQL into classes that are not part of the model | Will be available in a future version. |
| Connection resiliency support | The ability to retry connecting and sending queries | Will be available in a future version. https://github.com/aspnet/EntityFramework/issues/237 Implemented in EF Core 1.1. |
| Seeding data in migrations | The ability to add data when migrating | Will be available in a future version. |
| Command and query interception | The ability to intercept queries and SQL commands | Will be available in a future version. https://github.com/aspnet/EntityFramework/issues/626 |
| Visual Studio support for generating/updating entities from the database and viewing the model | The ability to generate the model from the database from inside Visual Studio and to view the model graphically | Will be available in a future version. |
| Database initializers | Database initializers | Dropped. |
| Automatic migrations | Automatic migrations | Dropped. |
| Pluralization Service | Pluralization Service | Dropped. |
| ObjectContext events | SavingChanges and ObjectMaterialized events of ObjectContext | Dropped. |
| ObjectContext (Entity SQL) | Entity SQL | Dropped. |
| Model first approach | Model first approach | Dropped. |
| Data Annotations validations | The ability to perform data annotations validations before saving changes | Dropped. |
Please let me know if you think I missed something! 
For the most up to date roadmap, please consult the EF Core roadmap: https://github.com/aspnet/EntityFramework/wiki/Roadmap.
Also, do check out the Entity Framework issue tracker: https://github.com/aspnet/EntityFramework/issues