Have you ever noticed how recipe writers (and especially the hosts of television cooking programs) get caught up when they instruct you to "deglaze the pan?" They become obsessed with dissolving and scraping up all the "brown bits" that have stuck to the bottom of the pan. Some go so far as to make the radical statement that, "that's where all the flavor is."
Well, to a great extent, they're right. Raw meat has very little flavor (raw chicken is said to taste metallic; raw beef, bland but tasting of blood). In cooking meat and many other foods, a series of complex chemical reactions sometimes occur - the Maillard reactions. These take place when amino acids react with sugars in the food, and they break down relatively large, stable, and - from the point of view of flavor - dull molecules into smaller, volatile molecules that we can easily smell and taste. Peter Barham, author of The Science of Cooking (Canada, UK), says, "the majority of the flavour comes from the small molecules in the food." The Maillard reactions are also what cause the meat to brown.
Until these Maillard reactions occur, however, meat continues to have relatively little flavor. And Maillard reactions, which are finicky and not completely understood by scientists, generally only begin to occur above 285°F (140°C). Now, if you're braising a piece of meat or cooking it in a stew, the temperature in the pot will never get above the boiling point of water (212°F or 100°C), so the Maillard reactions will never take place, and your meat will be cooked but will have relatively little flavor. This is why you are almost always instructed to brown the meat on all sides before adding it to the stew.
Similarly, slow-cooking a roast in a low-temperature oven fails to set off the Maillard reactions, which is why some recipes encourage you to raise the temperature of the oven dramatically in the last 15 or 20 minutes of cooking, so that the roast browns and develops the flavor you expect from an expensive cut of meat.
The Maillard reactions occur only at the surface of the meat, because the moisture in the meat keeps the interior from getting above 212°F. This is why some recipes encourage you to cut the meat into smaller pieces, to expose more surface area to the higher temperatures and produce exponentially more Maillard reactions. This is also why you often marinate or stud with garlic or douse in gravy larger roasts that you did not cut up before cooking - to impart flavor to the portions below the surface that will not benefit from the scientific and culinary discoveries of Monsieur Maillard.
This is also why the "brown bits" on the bottom of your roasting pan are so valuable. They do indeed carry much flavor, and if you throw them out - as we once saw someone do as he began to make gravy with a bouillon cube - the only flavor you're likely to have will come from your tears splashing onto the plate (as did ours, as we began to eat the bouillon-based gravy).