I'm no doc, but I like reading up on this type of stuff even if a good chunk of it goes over my head. This is from a book put out by Men's Health that I just read:
"Protein is made of amino acids....The best protein sources contain all nine of the essential amino acids in relatively the same amounts that your body requires. The next best contain all of the essential aminos, but a few of the values fall below your body's needs. The worst proteins are either missing an amino altogether or provide low values of most or all of the essential aminos."
Then it lists some of the BEST as "dairy products (especially cottage cheese), eggs, beef, pork, poultry, fish, oats, nuts, and soy protein," NEXT BEST including "beans, seeds, and cornmeal" and WORST "White or wheat breads, peas, rice, potatoes, pasta and gelatin".
"At this point, you may wonder why vegetarians don't collapse into gelatinous heaps from eating foods missing essential aminos. That's because your body can combine incomplete proteins to make complete ones. The only problem with this strategy is that relying on incomplete proteins can lead to incomplete muscle development. We don't mean that the muscles themselves will be incomplete - your vegan friend isn't missing half a biceps. He just won't grow as much muscle as we meat eaters do, when all else is equal."
This is all from The Testosterone Advantage Plan, a Men's Health book by Lou Schuler and some other people. Like I said, I'm no doc, but Men's Health stuff tends to make pretty concise, easy to read stuff that makes more sense to me than if I read the studies or did the research myself. If anyone knows better than this stuff, though, I'd love to know about it.
From what I got out of the book, the meat v. other sources of protein thing is mostly an efficiency issue. That being said, I eat pretty much everything. I think it's interesting that the book is written assuming the reader isn't a vegetarian, too. Hope this was helpful some.