For example, everyone knows butter and margarine are high in fat - why not replace with low fat spreads or polyunsaturated margarine?

Low fat recipes won't take more time to prepare than any other types of recipes either. They're just as simple, and low fat recipes are an important foundation to healthy cooking, healthy eating, and a healthy lifestyle.
Simple fat knowledge: the fats in food are made up of blocks of fatty acids and glycerol. The fatty acids are chains of carbon atoms, and the type of fat is determined by how these chains link together. There are three types of fat: saturated, polyunsaturated, and monounsaturated fats, and all three are present in the foods we eat.
Fats are a concentrated source of energy. The daily intake of fat shouldn't exceed 30% of total calories. So, for an average daily intake of 2,000 calories, only 600 should come from fat.
For a lowfat diet, you can easily to cut down the most common sources of fat - such as: butter, cheese, cream, full fat milk, fatty meats, pastries, etc...
That's not to say that you should completely avoid these types of foods. That would just be unconscionable. Go ahead and splurge sometimes - cooking low fat recipes regularly will allow you to do so without your food boring you out of your mind. Low fat cooking recipes can be just as tasty as the 'fatty' dishes!