The tikz option of standalone generates one page for all tikzpictures it finds, because it also enables the multi option. For this it will box and discard everything which is outside of such environments.
It is not intended to be used with \tikz inside a tabular. I recommend to drop the tikz option and load the tikz package manually instead.
\documentclass{standalone}
\usepackage{tikz}
\begin{document}
\tikzset{every path/.style={line width=0.66em}}
\begin{tabular}{ll}
$<$ 1 hour & \tikz \draw (0,0) -- (2.874cm,0) node[anchor=west] {28.74\%};
\end{tabular}
\end{document}
\documentclass{standalone} \usepackage{tikz}instead of\documentclass[tikz]{standalone}fixes the problem. – Peter Grill Sep 16 '12 at 17:28