o   Here-documents within $(...) command substitutions may once more be
    delimited by the closing right paren, instead of requiring a newline.

o   Bash's file status checks (executable, readable, etc.) now take file
    system ACLs into account on file systems that support them.

o   Bash now passes environment variables with names that are not valid
    shell variable names through into the environment passed to child
    processes.

o   The `execute-unix-command' readline function now attempts to clear and
    reuse the current line rather than move to a new one after the command
    executes.

o   `printf -v' can now assign values to array indices.

o   New `complete -E' and `compopt -E' options that work on the "empty"
    completion: completion attempted on an empty command line.

o   New complete/compgen/compopt -D option to define a `default' completion:
    a completion to be invoked on command for which no completion has been
    defined.  If this function returns 124, programmable completion is
    attempted again, allowing a user to dynamically build a set of completions
    as completion is attempted by having the default completion function
    install individual completion functions each time it is invoked.

o   When displaying associative arrays, subscripts are now quoted.

o   Changes to dabbrev-expand to make it more `emacs-like': no space appended
    after matches, completions are not sorted, and most recent history entries
    are presented first.

o   The [[ and (( commands are now subject to the setting of `set -e' and the
    ERR trap.

o   The source/. builtin now removes NUL bytes from the file before attempting
    to parse commands.

o   There is a new configuration option (in config-top.h) that forces bash to
    forward all history entries to syslog.

o   A new variable $BASHOPTS to export shell options settable using `shopt' to
    child processes.

o   There is a new confgure option that forces the extglob option to be
    enabled by default.

o   New variable $BASH_XTRACEFD; when set to an integer bash will write xtrace
    output to that file descriptor.

o   If the optional left-hand-side of a redirection is of the form {var}, the
    shell assigns the file descriptor used to $var or uses $var as the file
    descriptor to move or close, depending on the redirection operator.

o   The < and > operators to the [[ conditional command now do string
    comparison according to the current locale.

o   Programmable completion now uses the completion for `b' instead of `a'
    when completion is attempted on a line like: a $(b c.

o   Force extglob on temporarily when parsing the pattern argument to
    the == and != operators to the [[ command, for compatibility.

o   Changed the behavior of interrupting the wait builtin when a SIGCHLD is
    received and a trap on SIGCHLD is set to be Posix-mode only.

o   The read builtin has a new `-N nchars' option, which reads exactly NCHARS
    characters, ignoring delimiters like newline.

o   The mapfile/readarray builtin no longer stores the commands it invokes via
    callbacks in the history list.

o   There is a new `compat40' shopt option.

o   The < and > operators to [[ do string comparisons using the current locale
    only if the compatibility level is greater than 40 (set to 41 by default).

o   New bindable readline function: menu-complete-backward.

o   In the readline vi-mode insertion keymap, C-n is now bound to menu-complete
    by default, and C-p to menu-complete-backward.

o   When in readline vi command mode, repeatedly hitting ESC now does nothing,
    even when ESC introduces a bound key sequence.  This is closer to how
    historical vi behaves.

o   New bindable readline function: skip-csi-sequence.  Can be used as a
    default to consume key sequences generated by keys like Home and End
    without having to bind all keys.

o   New bindable readline variable: skip-completed-text, active when
    completing in the middle of a word.  If enabled, it means that characters
    in the completion that match characters in the remainder of the word are
    "skipped" rather than inserted into the line.

o   The pre-readline-6.0 version of menu completion is available as
    "old-menu-complete" for users who do not like the readline-6.0 version.

o   New bindable readline variable: echo-control-characters.  If enabled, and
    the tty ECHOCTL bit is set, controls the echoing of characters
    corresponding to keyboard-generated signals.

o   New bindable readline variable: enable-meta-key.  Controls whether or not
    readline sends the smm/rmm sequences if the terminal indicates it has a
    meta key that enables eight-bit characters.