How does zsh expand variables before passing to programs?

Question

A basic rule of zsh is that you don't need to quote your variables, for example:

% data="single argument"
% print -l $data
single argument

One exception I know of is that if an argument must not be skipped even if empty, it must be double quoted:

% emptyarg=
% functon count() { echo $# }
% count $emptyarg
0
% count "$emptyarg"
1

However, if an argument contains certain special characters, it must be quoted or the shell gives an error. Why is that? It seems fragile. The content of my data shouldn't substantially influence how functions/commands run.

Answer 1

Answering my own question-- I noticed the behavior works as expected in scripts (parameters don't expand, even without quotes). I realized the issue was probably caused by an option, so I ran setopt in my interactive shell and in a script and compared the results. I turned them off one by one until I found setopt noglobsubst had the desired effect. In short, the GLOB_SUBST option makes zsh treat all variables as patterns to be expanded if possible. You can disable this option and still expand variables when needed with the tilde ~ parameter expansion:

% setopt noglobsubst
% star=*
% echo $star
*
% echo $~star
readme.txt test.sh