Unfortunately these changes aren't summarized. The differences are all the 'feature' bullets in the changes webpage from 1.1.0 to 1.2.0. I've assembled these below:

  - the $connection_requests variable.
  - $tcpinfo_rtt, $tcpinfo_rttvar, $tcpinfo_snd_cwnd and $tcpinfo_rcv_space variables.  
  - the "worker_cpu_affinity" directive now works on FreeBSD.
  - the "xslt_param" and "xslt_string_param" directives.  Thanks to Samuel Behan.  
  - the "from" parameter of the "disable_symlinks" directive.
  - the "return" and "error_page" directives can now be used to return 307 redirections.  
  - the "disable_symlinks" directive.
  - the "proxy_cookie_domain" and "proxy_cookie_path" directives.  
  - multiple "limit_req" limits may be used simultaneously.
  - the "TLSv1.1" and "TLSv1.2" parameters of the "ssl_protocols" directive.  
  - the "proxy/fastcgi/scgi/uwsgi_cache_lock", "proxy/fastcgi/scgi/uwsgi_cache_lock_timeout" directives.  
  - the "pcre_jit" directive.
  - the "if" SSI command supports captures in regular expressions.  
  - the "so_keepalive" parameter of the "listen" directive.  Thanks to Vsevolod Stakhov.  
  - the "if_not_empty" parameter of the "fastcgi/scgi/uwsgi_param" directives.  
  - the $https variable.
  - the "proxy_redirect" directive supports variables in the first parameter.
  - the "proxy_redirect" directive supports regular expressions.  
  - the "valid" parameter of the "resolver" directive. By default TTL returned by a DNS server is used.
  - support for multiple "limit_conn" limits on the same level.  
  - the "image_filter_sharpen" directive.  
  - support of several DNS servers in the "resolver" directive.  Thanks to Kirill A. Korinskiy.
  - "proxy/fastcgi/scgi/uwsgi_ignore_headers" directives support the following additional values: X-Accel-Limit-Rate,
  - decrease of memory consumption if SSL is used.  
  - the "uwsgi_buffering" and "scgi_buffering" directives.  Thanks to Peter Smit.
  - the ngx_http_upstream_keepalive module.  
  - the "proxy_http_version" directive.  
  - the "fastcgi_keep_conn" directive.  
  - the "worker_aio_requests" directive.  
  - the module ngx_http_mp4_module.  
  - the "max_ranges" directive.  
  - now shared zones and caches use POSIX semaphores on Solaris.  Thanks to Den Ivanov.
  - accept filters are now supported on NetBSD.  
  - cache loader run time decrease.  
  - "loader_files", "loader_sleep", and "loader_threshold" options of the "proxy/fastcgi/scgi/uwsgi_cache_path" directives.
  - loading time decrease of configuration with large number of HTTPS sites.
  - now nginx supports ECDHE key exchange ciphers.  Thanks to Adrian Kotelba.
  - the "lingering_close" directive.  Thanks to Maxim Dounin.