Here's a pretty well-documented summary from Eat Wild (http://www.eatwild.com/healthbenefits.htm):
Summary of Important Health Benefits of Grassfed Meats, Eggs and Dairy
Lower in Fat and Calories. There are a number of nutritional differences between the meat of pasture-raised and feedlot-raised animals. To begin with, meat from grass-fed cattle, sheep, and bison is lower in total fat. If the meat is very lean, it can have one third as much fat as a similar cut from a grain-fed animal. In fact, as you can see by the graph below, grass-fed beef can have the same amount of fat as skinless chicken breast, wild deer, or elk. Research shows that lean beef actually lowers your "bad" LDL cholesterol levels.
Because meat from grass-fed animals is lower in fat than meat from grain-fed animals, it is also lower in calories. (Fat has 9 calories per gram, compared with only 4 calories for protein and carbohydrates. The greater the fat content, the greater the number of calories.) As an example, a 6-ounce steak from a grass-finished steer can have 100 fewer calories than a 6-ounce steak from a grain-fed steer.
(For more information about Missouri Beef as Best Health Food, see below pdf...)