Enhancements
[ExUnit] Add on_exit/1 callbacks that are guaranteed to run once the test process exits and always in another process
[Kernel] Store documentation in the abstract code to avoid loading them when the module is loaded
[Kernel] Add get_in/2, put_in/3, update_in/3 and get_and_update_in/3 to handle nested data structure operations
[Kernel] Add get_in/1, put_in/2, update_in/2 and get_and_update_in/2 to handle nested data structure operations via paths
[Mix] Add Mix.Config to ease definition of configuration files
[Mix] Add mix loadconfig task that can be called multiple times to load external configs
[Mix] Support --config option on mix run
[Mix] Support HTTP_PROXY and HTTPS_PROXY on Mix url commands
[Mix] Support --names options in mix help which emit only names (useful for autocompletion)
[Protocol] Add Protocol.consolidate/2, Protocol.consolidated?/1 and a mix compile.protocols task for protocol consolidation
[Protocol] Add Protocol.derive/3 for runtime deriving of a struct
[String] Add String.chunk/2
[Struct] Add support for @derive before defstruct/2 definitions
Bug fixes
[File] File.rm now consistently deletes read-only across operating systems
[Kernel] Ensure Mix _build structure works on Windows when copying projects
[Kernel] Ensure 1.0E10 (with uppercase E) is also valid syntax
[Mix] Fix mix do task for Windows' powershell users
[Path] Fix Path.absname("/") and Path.expand("/") to return the absolute path "/".
Soft deprecations (no warnings emitted)
[Kernel] size/1 is deprecated, please use byte_size/1 or tuple_size/1 instead
[ExUnit] teardown/2 and teardown_all/2 are deprecated in favor of on_exit/1 callbacks
Deprecations
[Access] Access.access/2 is deprecated in favor of Access.get/2
[Dict] Dict.Behaviour is deprecated in favor of Dict
[Kernel] Application.Behaviour, GenEvent.Behaviour, GenServer.Behaviour and Supervisor.Behaviour are deprecated in favor of Application, GenEvent, GenServer and Supervisor
[Kernel] defexception/3 is deprecated in favor of defexception/1
[Kernel] raise/3 is deprecated in favor of reraise/2
[Kernel] set_elem/3 is deprecated in favor of put_elem/3
[Kernel] Passing an atom var!/1 is deprecated, variables can be built dynamically with Macro.var/2
[Mix] Exceptions that define a :mix_error field to be compatible with Mix are no longer supported. Instead please provide a :mix field and use Mix.raise/1 and Mix.raise/2
Backwards incompatible changes
[Access] Kernel.access/2 no longer exists and the Access protocol now requires get/2 (instead of access/2) and get_and_update/3 to be implemented
[Kernel] Retrieving docs as module.__info__(:docs) is no longer supported, please use Code.get_docs/2 instead
[Kernel] Code.compiler_options/1 no longer accepts custom options, only the ones specified by Elixir (use mix config instead)
[Mix] mix new no longer generates a supevision tree by default, please pass --sup instead
[Task] Tasks are automatically linked to callers and a failure in the task will crash the caller directly
But overall, even for a new fast moving language, the stability of erlang benefits it pretty well.