In the short term, meal replacement shakes can be a useful tool to control portion sizes, jump start a diet and lead to weight loss. However, there is not great deal of research regarding long term use  and I would not recommend them for other than occasional or convenience use. Here are some of my concerns:
- Satiety. Research consistently shows that drinking calories does not provide the same sense of fullness as eating equivalent calories.
- Sugar and Sugar Substitutes. Many meal replacement shakes are artificially sweetened or contain a lot of added sugar. This not only adds calories, but keeps your palate attuned to a high level of sweetness.
- Fructose (fruit sugar) is the third ingredient in the IsaLean Chocolate and Vanilla shakes and seems to be the main source of carbohydrate provided. Each serving has 28g of carbohydrate, half of which is from sugar.
- Even well-thought-of GoLean Kashi Chewy bars have several types of added sugars: brown rice syrup, pear juice concentrate, malt syrup, evaporated cane juice and more!
- Artificial ingredients. Most meal replacements are loaded with added flavorings.
- Lack of variety. Real food is more than the sum of its nutrient parts. There are possibly thousands of antioxidant compounds in food - many as yet unidentified. When a meal replacement is formulated, the manufacturer must choose exactly what nutrients it will contain. If you eat the same meal replacements day after day, your nutrient intake hardly varies. How does the manufacturer know what is best for your health over months or years? Do you trust them to decide that for you? There is good evidence that a diet rich in vegetables, fruits, whole grains and unsaturated fats has health benefits. In contrast, we just don't know much about long-term meal replacement use. When liquid formulas are used for medical reasons (as in a person who cannot chew or swallow), the goal is always to get the patient back on real food.
OK, with that said, if you continue to use meal replacement shakes, here are some considerations:
If you are trying to build muscle, whey protein is a good choice. As a "fast" protein, whey is digested quickly. When eaten within 60 minutes of finishing a workout, whey protein should increase your blood supply of amino acids and stimulate protein synthesis. The amount needed is not large; one study showed only 10g of whey protein and 20g of carbohydrate was effective in building muscle . The body cannot store large reserves of protein. Unless you consume it directly following a workout - when your muscles should be primed for amino acid uptake - excess protein will be burned for energy or converted to fat, just like any other calories. Since whey protein powders can be expensive, consider saving it for those post-workout periods when it will you do the most good. Egg whites and milk are also high-quality protein sources - for a cheaper price.
I know you asked what to look for in a shake. Honestly, I am not quite sure how to answer, since I'm not a fan of any of them. I'd say try to look for products with the least amount of artificial ingredients, flavorings and sweeteners. I'd prefer something simple like the IsaPro whey protein powder, which is made to be added to something else. You could create your own meal replacements out of real foods like fruit, berries, yogurt, peanut butter, vegetables, etc. Make more than you need, portion it out and keep in the fridge or freezer for later.
[A general service message about protein: the typical American eats much more protein than s/he needs. For a healthy adult, protein requirements are 0.8g per kilogram of body weight. For a 150 lb (68kg) person, that's 54.4g of protein per day. To put this in perspective, 1oz of cooked chicken breast has approximately 8g of protein. It doesn't take much to meet the requirement. If you have healthy kidneys, a higher-protein diet should not be a problem (my diet tends to be on the high-protein side). However, very high protein diets (upwards of 2.5g protein per kg of weight) may contribute to dehydration and bone loss .]
If the title of this blog hasn't given it away already: I am a foodist. I will always favor real food. Try as we might, we just can't replicate the whole spectrum of naturally-occurring nutrients in bar, shake or supplement form - not to mention the pleasure that comes from eating a tasty meal. In the long run, meal-replacement users probably will not develop any nutrient deficiencies, but I think they will be short-changed out of the many beneficial substances found only in real food.
 Spahn, J.M., Reeves, R.S., Keim, K.S., Laquatra, I., Kellogg, M., Jortberg, B., Clark, N.A. (2010). State of the evidence regarding behavior change theories and strategies in nutrition counseling to facilitate health and food behavior change. Journal of the American Dietetic Association. 110, 879-891.
 Rosenbloom, C. (2009) Protein for athletes: quantity, quality, and timing. Nutriton Today. 44(5):204–210.