Classic skis intended for racing or high performance are going to be lighter are generally skinnier than 'sport' or recreational skis. Race skis generally have a taller, stiffer camber that requires better technique to engage the grip area, and usually have waxable bases with a hard, fast base material.
Classic skis generally have a unique 'double camber' construction that prevents the underfoot grip area on the ski from making contact with the snow when you're gliding downhill with your weight distributed over both skis. When you're striding in the flats, you tend to place your entire weight on one ski at a time, which is enough to engage the grip area. It is important to size your ski correctly in order to allow this feature to work properly.
If you already have or intend to buy NNN/NIS bindings and boots, look for a ski with an NIS plate mounted to it. If you want to keep your options open, look for one with a plain flat top. On the underside, bases are either waxable (you apply kick wax to get the grip needed for uphill) or waxless (the grip area is built in, either with 'fish scales' or small 'skin' sections).