http://globaldev.co.uk/2014/05/ruby-2-1-in-detail

• New versioning policy
• Required keyword arguments
• String#freeze optimisation
• def returns the method name as a Symbol
• Rational and Complex literals
• Array/Enumerable #to_h
• Fine grained method cache
• Exception
• Generational Garbage Collection
• GC
• GC tuning environment variables
• ObjectSpace tools to track down memory leaks
• Refinements
• String#scrub
• Bignum/Rational performance improvements
• $SAFE level 4 removed
• clock_gettime
• RubyGems updated
• Deprecated Rake features removed
• RDoc template update
• Process title
• Frozen Symbols
• Fixed eval scope leak
• #untrusted? is now an alias of #tainted?
• return in lambda now always returns from lambda
• Get interface addresses
• Named capture support in StringScanner
• YAML.safe_load
• Resolv one-shot MDNS and LOC record support
• Improved Socket error messages
• Hash#shift much faster
• Queue, SizedQueue, and ConditionVariable performance improvements
• Timeout internal exception can’t be rescued
• Easier streaming responses with WEBrick
• Numeric#step
• Use IO _nonblock without raising exceptions
• IO ignores internal encoding if external encoding is ASCII-8BIT
• #include and #prepend now public
• Module/Class #singleton_class?
• Module#ancestors more consistent
• Object#singleton_method
• Method#original_name
• Mutex#owned?
• Hash#reject
• Vector#cross_product
• Fixnum/Bignum #bit_length
• pack/unpack Native Endian long long
• Dir glob returns composed characters
• Better type coercion for Numeric#quo
• Binding#local_variable_get/_set/_defined?
• CGI class methods now available from CGI::Util module
• Digest::Class.file passes arguments to initialiser
• Net::SMTP#rset
• open-uri supports repeated headers
• Write to files with Pathname
• Tempfile.create
• Rinda multicast support
• Easy setting of extra HTTP options for XMLRPC
• URI.encode_/decode_www_form updated to match WHATWG standard
• RbConfig::SIZEOF
• Can set facility with Syslog::Logger
• CSV.foreach with no block returns working enumerator
• OpenSSL bignum
• Enumerator.new size argument fixed to accept any callable object
• curses library removed
• TSort class methods
• TCP Fast Open