\documentclass{article}

\begin{document}

\begin{itemize}

\item Lists with items on one line

\item Lists with items
on multiple lines

% comments before a list item
\item Another item

\item Another item
% comments inside a list item
Or even just % trailing comments

\item Every \item should start \item a new line

\end{itemize}

\begin{myitemize}

\item Custom list environments can be specified in the configuration file.

\end{myitemize}

Commands such as itemsep should not be affected.
\setlength{\itemsep}{0pt}

\end{document}
