The most basic saddle adjustment is the height. Most bicyclists have their saddles too low, so that their knees are excessively bent as they pedal. This makes cycling much more tiring for a given speed, and is likely to cause harm to the knees.

A common reason for keeping the saddle set too low is that most bicyclists have never learned the proper technique for mounting and dismounting, so they find it convenient to be able to put a foot down to steady the biycle while they are stopped. With older bicycles, it was sometimes possible to put a toe down at a stop with the saddle properly adjusted, especially for riders with large feet. Due to the higher bottom brackets common on newer bicycles, especially mountain bikes, it is no longer possible to do this. (my emphasis-maptester) If you ride a mountain bike, and are able to balance it while stopped and seated, it is a sure sign that your saddle is too low. This is also true of most hybrids.

Having the saddle too low makes it harder to carry much of your weight on your legs, so you will sit with more weight on the saddle. This, in itself, is likely to increase saddle discomfort.