The major effect of too much food is fat. It doesn't matter whether you're getting your calories from fats, carbohydrates (complex or simple), proteins, alcohols--if you consume more calories in a day than you burn, your body will store the excess as fat. The reverse is true as well--regardless of the source of your calories, consuming fewer than you burn will lead to weight loss. There are healthier and less healthy ways to go about this, and you need at least some of each of the major calorie sources, but many people put a lot of effort into complicated schemes of exact balancing of carbs, proteins, fats, etc, when they could save a lot of effort and get pretty much the same results just by dealing more with portion control.
The secondary effect of too much protein is kidney and liver problems. Kidney stones have already been mentioned, but other complications can arise. Your kidneys and liver primarily function to detoxify your blood, and too much protein is toxic. Thus, they work in overdrive dealing with the excess amino acids, and can get damaged as a result. This is one of the reason why native Inuit have high rates of kidney and liver disease, even while maintaining their traditional diet.
Also, as a point of reference, only some vitamins will simply pass through you when taken in excess--the water soluble ones. The fat soluble ones can build up in your system and causes toxic problems of their own. In general, it's better to get 100% of the FDA daily recommended allowance of something than 300%, even if it's a vitamin. More isn't always better.
