Since the 1960s and 1970s, researchers have proposed simplified models using only a few parameters to describe the vibro-acoustical behavior of string instruments in the low-frequency range. This paper presents a method for deriving and estimating a few important parameters or features describing the mechanical behavior of classical guitars over a broader frequency range. These features are selected under the constraint that the measurements may readily be made in the workshop of an instrument maker. The computations of these features use estimates of the modal parameters over a large frequency range, made with the high-resolution subspace ESPRIT algorithm (Estimation of Signal Parameters via Rotational Invariant Techniques) and the signal enumeration technique ESTER (ESTimation of ERror). The methods are applied to experiments on real metal and wood plates and numerical simulations of them. The results on guitars show a nearly constant mode density in the mid- and high-frequency ranges, as it is found for a flat panel. Four features are chosen as characteristic parameters of this equivalent plate: Mass, rigidity, characteristic admittance, and the mobility deviation. Application to a set of 12 guitars indicates that these features are good candidates to discriminate different classes of classical guitars.