if you're thinking about cakes, you can reduce the sugar of many recipes considerably.
compare the amount of flour with the amount of sugar. if the sugar is less than the flour, it's known as a low-ratio cake (although low-ratio can also mean the percentage of fat compared to flour, too). if the sugar is more than the flour, it's a high-ratio cake.
low-ratio cakes can go down to around 75% sugar as compared to flour, or to say another way, 3/4 cup sugar per 1 cup flour. some high-ratio cakes can go over double the amount of sugar as compared to flour, or 2 cups sugar to every 1 cup flour.
so, you can generally reduce a high-ratio recipe to say equal amounts of flour and sugar, and you probably wouldn't consider it to be too
sugar adds not only sweetness, but moisture as well. if you reduce the amount of sugar by a large percentage, you might want to increase the fat or liquid (butter, milk, liqueur, etc.) by a tablespoon or two.
my standard genoise recipe calls for:
1 c sugar
1 c flour
1/2 c clarified butter
1 teaspoon vanilla
if you substitute half of the flour with cocoa powder, it's not a particulary sweet cake in the least, as is. as is, you could probably reduce the sugar by an ounce without any problem.