One of the biggest new features in SQL Server 2014 is SQL Server In-Memory OLTP, known mostly by its code name of Hekaton (see SQL Server 2014: In-memory OLTP Engine, code-name: Hekaton). Basically, this puts tables in memory, giving a potentially huge performance boost when accessing these tables, especially with writes (to perform your own performance testing, check out SQL Server 2014 CTP2 In-Memory OLTP Sample). But be aware that there are currently many features that In-Memory OLTP does not support:
For memory-optimized tables, some of the features not supported are: IDENTIFY or SEQUENCE columns, foreign key constraints, check constraints, MERGE statement, DML triggers, XML/CLR/LOB data types, compression.
Also, with In-Memory OLTP you can natively compile stored procedures (with native_compilation), but the stored procedures can only access in-memory tables. And there are many features that you cannot use inside a natively compile stored procedure, such as: cursors, CASE statement, CTEs, user-defined functions, UNION statement, PIVOT statement, DISTINCT statement, and many more.
See Transact-SQL Constructs Not Supported by In-Memory OLTP for a comprehensive list of what is not supported.