9 goes quite far. My most common uses are downloads/*(oc[1]) for the latest downloaded file (order by time of inode change and return just first result), or downloads/*(c0) for all the files downloaded today (created 0 days ago in downloads directory).
10) =() expansion. It's like 11) short syntax for control structures. For example, you can do `for x in $(xs); foo $x` without bothering with `do` and `done`.
12) variable expansion in brace expansion. In bash, `x=5; echo {3..$x}` outputs `{3..5}`. zsh outputs `3 4 5`.
13) Command can be just redirection without any executable. ` foo.txt` is equivalent to `cat > foo.txt`.
14) Able to convert numbers between any bases. To convert base-5 10 to base-4: `15) Named directories. I have my project files organized under deep directories like ~/work-for/.../.../.../foo/, and have configured all those as named directories to be able to access them as `~foo`, for instance. Different from using simple variables for this, named directories are compacted when presented by the shell. So in my prompt, I see my current directory is e.g. `~foo/bar` instead of the whole thing from the home dir. Named directories come in 2 forms, static and dynamic. Static named dirs are when you set them up in a hash of name => directory. Dynamic named dirs are when you define a function that expands arbitrary names to paths and compacts arbitrary paths to names.
16) `=foo` expands to the full path of executable foo. So you can, e.g. see the source of `pass` with `less =pass`.
17) Variable expansion syntax, glob qualifiers, and history modifiers can be combined/nested quite nicely. For example, this outputs all the commands available from $PATH: `echo ${~${path/%/\/*(*N:t)}}`. `${~foo}` is to enable glob expansion on the result of foo. `${foo/%/bar}` substitutes the end of the result of foo to "bar" (i.e. it appends it); when foo is an array, it does it for each element. In `/*(*N:t)`, we're adding the slash and star to the paths from `$path`, then the parentheses are glob qualifiers. `*` inside means only match the executables, `N` is to activate NULL_GLOB for the match so that we don't get errors for globs that didn't match anything, `:t` is a history mod used for globs that returns just the "tail" of the result, i.e. the basename. IIRC, bash can't even nest multiple parameter expansions; you need to save each step separately.
18) Suffix aliases can be used to define how to open arbitrary files. `./foo.mp4` opens it up in with `mpv`, because I have `alias -s mp4=mpv`.
19) Global aliases are useful for expanding stuff anywhere. For example, I have `alias -g %LD='~/downloads/*(oc[1])'` so that `%LD` expands to the latest download as part of any argument to a command.
20) Globbing flags. There are various flags for globbing, that enable e.g. backreferences in globs, allow n-number of errors while matching (approximate matching), etc. The only one I've really used though is `i` which enables a glob to be case-insensitive. So `(#i)*.pdf` matches `foo.pdf` as well as `bar.PDF`.
21) Multios. You can use multiple files in individual redirections. For example, `foo > bar | baz` is equivalent to `foo | tee bar | baz`. That accepts globbing and brace expansion too: `foo > *.{c,h}` is the same as `foo | tee *.{c,h}`. Also works for reading: `foo < *.txt` is practically the same as `cat *.txt | foo`. I must admit that I haven't really used this, though.