One way is to increase the width of the lines by
\setlength\arrayrulewidth{1pt}
Code:
\documentclass[english,a4paper,12pt]{report}
\usepackage[T1]{fontenc}
\usepackage{graphicx}
\usepackage[latin9]{inputenc}
\usepackage{graphicx}
\usepackage{xcolor,colortbl}
\usepackage[showframe=false]{geometry}
\usepackage{changepage}
\usepackage{hhline}
\usepackage[font=normalsize,labelfont=bf,justification=centering]{caption}
\begin{document}
\begin{table}[htb]
\setlength\arrayrulewidth{1pt} %% put it inside table environment so that change is local
\begin{adjustwidth}{-1cm}{}
\resizebox{0.6\textwidth}{!}{
\begin{minipage}{\linewidth}
\def\arraystretch{1.5}%
\begin{tabular}{ | c | c | c | c | c | c | c | c | c | c | c | }
\cline{2-11}
\multicolumn{1}{c|}{} & \textbf{Telestrada} & \textbf{Telestrada} & \textbf{Telestrada} & \textbf{Telestrada} &
\textbf{Telestrada} & \textbf{Telestrada} & \textbf{Telestrada} &
\textbf{Telestrada} & \textbf{Telestrada} & \textbf{Telestrada} \\
\hline
\textbf{LOC} & 5 & 5 & 6 & 5 & - & 5 & 6 & 5 & 6 & 5 \\
\hline
\rowcolor{gray!40} \textbf{VS} & 8 & 5 & 9 & 8 & 9 & 8 & 9 & 8 & 9 & 8 \\
\hline
\end{tabular}
\end{minipage}
}
\end{adjustwidth}
%% put caption outside here
\caption{\label{fig:text}text experiment experiment experiment experiment experiment experiment experiment}
\end{table}
\end{document}

Your tables become more readable and beautiful if you get rid of vertical lines and some of the horizontal lines (you need only three of them, two at the top and one at the bottom). We do this with booktabs
package (whose documentation explains some good practices about writing tables)
\documentclass[english,a4paper,12pt]{report}
\usepackage[T1]{fontenc}
\usepackage{graphicx}
\usepackage[latin9]{inputenc}
\usepackage{graphicx}
\usepackage{xcolor,colortbl}
\usepackage[showframe=false]{geometry}
\usepackage{changepage}
\usepackage{hhline}
\usepackage{booktabs}
\usepackage[font=normalsize,labelfont=bf,justification=centering]{caption}
\begin{document}
\begin{table}[htb]
\begin{adjustwidth}{-1cm}{}
\resizebox{0.6\textwidth}{!}{
\begin{minipage}{\linewidth}
\def\arraystretch{1.5}%
\begin{tabular}{ *{11}{c}}
\\\toprule
& \textbf{Telestrada} & \textbf{Telestrada} & \textbf{Telestrada} & \textbf{Telestrada} &
\textbf{Telestrada} & \textbf{Telestrada} & \textbf{Telestrada} &
\textbf{Telestrada} & \textbf{Telestrada} & \textbf{Telestrada} \\
\midrule
\textbf{LOC} & 5 & 5 & 6 & 5 & - & 5 & 6 & 5 & 6 & 5 \\
\rowcolor{gray!40} \textbf{VS} & 8 & 5 & 9 & 8 & 9 & 8 & 9 & 8 & 9 & 8 \\
\bottomrule
\end{tabular}
\end{minipage}
}
\end{adjustwidth}
%% put caption outside here
\caption{\label{fig:text}text experiment experiment experiment experiment experiment experiment experiment}
\end{table}
\end{document}

booktabs
where these are distinctly visible. If you want I will provide some code. – user11232 Mar 30 '14 at 22:43