Wicking ability is what makes synthetic fabrics work so much better than cotton. Whereas cotton will simply absorb moisture and hold it, synthetics will move the moisture to the outside of the fabric, where it quickly evaporates. Most long underwear fabrics have been treated with some sort of chemical that aids in wicking. Polypropylene longies can still be found if you look hard. Wool is another option.
Some people find wool itchy and hot.
To me, high-quality merino wool feels soft as a baby lamb. You'll find long johns in a variety of weights and thicknesses. For people who tend to sweat a lot, lightweight fabrics are the ticket because they wick the fastest. Lightweights are also great for warm-weather outings, where you want that first layer to keep you dry and take the chill off, but not make you melt. Midweights work well in spring and fall and for highly aerobic winter activities like cross-country skiing and running.