\hbar not displayed with fontspec on XeLaTeX/LuaLaTeX

The definition of hbar is quite encoding dependant. As fontspec changes the operator font it breaks.

You could redefine it if you don't want to switch to unicode-math:

\documentclass{article}
\usepackage{fontspec}

\begin{document}\pagestyle{empty}

\def\hbar{{\mathchar175\mkern-9muh}}
[$\hbar$]

\def\hbar{\mathit{^^^^0127}}

[$\hbar$]

\end{document}