Diet is a real complicated matter. Parts of it look simple. For example, weight loss requires that we expend more energy than we take in.
Piece of cake! Can we complicate matters? You bet! If the energy content of our intake is mostly sugars, then our body doesn't really have a lot of engines set up to process other elements of our diet. So if we suddenly reduce the sugar content of our diet, these other sources of energy become important. The body will adjust to the new mix of energy sources, but it will take some time.
Energy balance is probably the most basic element of our diet. However, there are substances which our body needs to function, for which we either require raw materials to produce them, or we need to ingest them as our body either can't produce enough, or it can't produce them at all. But as you seen above, it is possible to manipulate the diet in such a way, that even an apparently balanced diet can be upset. Vitamins are the most obvious example of things we require in our diet, but there are also amino-acids, minerals and certain kinds of fats which we require for healthy living. I can't ever remember reading about any necessary carbohydrates.
If we do not supply our body with all the building blocks it requires to get through daily living, eventually something will happen to reduce our fitness or well being. Their are proteins which we require, which are absent in almost all vegetable sources. Minerals (or the metal ion in them) are often used as the active sites in enzymes. If we lack the mineral, we can't make enough of some enzyme, and we suffer. Vitamins are even more complex molecules that we require in our diet.
Someone (I don't remember who), said that if we somehow managed to eat 80 different kinds of foods during the course of a week (and things like sugar and salt don't count), it is very unlikely that we would ever need to take supplemental vitamins. In the absence of that variety, a good daily multi-vitamin is probably useful. This is not to say that if we eat 80 different kinds of food every week, we will be okay; or that if we only eat 70 different kinds of food one week, we are going to be in trouble. But it does say that we shouldn't expect to eat macaroni and cheese all the time, and stay healthy. (Note: 80 different kinds of pasta doesn't count. :-)
Certainly vegetarians know of the importance of legumes. Most vegetables lack a certain group of amino acids, and without them (legumes), vegetarians can suffer from deficiencies.
The diet of most "western" peoples is way too high in fat content, especially saturated fats (from animals) and trans-fats (from hydrogenating vegetable oils). Some fats, such as the omega-3 and omega-6 fats found in fish and flax oil, are essential. It appears that our bodies need compounds which are most easily found in soybean products.
Our need for fats varies with age. A great deal of our body's hormones are constructed from fats. Puberty is one time of our lives where we don't necessarily want to impose a low fat diet. This is not to say the teenagers should eat a high fat diet. They just shouldn't be as low as adults.
Things like Canda's Food Guide, have a special place for dairy products. While it is true that dairy is a good source of many nutrients, especially calcium, it shouldn't be regarded as necessary. Humans, and some cats, are the only mammals that ingest dairy as adults. So, it is possible to get all the nutritients present in dairy somewhere else.
This sort of argument (looking at other mammals) applies for any apparent need of "meat" in our diet. Vegetarian mammals exist, some of which are very strong and/or fast. Meat is not a necessary part of our diet. Like dairy, it may be convenient.
But the best advice I can give now, is to try to eat a varied diet. Don't eat macaroni and cheese all the time.