Freshly Ground Beef vs. Store Bought

Like you said, the main benefit is control. I'd say the two main variables you're controlling for are amount of fat in the mixture and the tenderness and quality of the cuts used. Depending on the application, you might use a different mixture of meat. (For burgers, Alton Brown uses a 50/50 mixture of chuck and sirloin.)

Grinding your own could also be considered a play for increased food safety. If there are any bacteria on the surface of the cuts of beef that go into the grinder, they will be pretty well distributed throughout grind. The longer (and warmer) the ground beef is stored between when it is ground and cooked, the more chance that the bacteria could grow to sufficient numbers that they could do some serious harm to the consumer. This is why it's recommended that ground beef is cooked to a higher internal temperature than say a steak. In grinding, everything effectively becomes surface area so you have to cook a burger all the way through to be sure you've killed any bacteria.

If you're grinding you're own, you can make the interval between grinding and cooking arbitrarily short, so if you want to take your chances with a rare burger, this would probably be the best reason to grind your own. You're still running a risk in this case as any bacteria that were on the outside of your meat are now on the inside of your burger and won't be killed if the meat is left rare. You would just be trusting that the butcher did a good job of keeping the outside of the cuts you purchased relatively free from infection. Also, any food safety benefit assumes you're doing a good job of cleaning your equipment. Meat grinders can be a real PITA to clean well.

As far as the flavor difference is concerned, I would assume that to be minimal, again if you control for any difference in quality and cuts of beef that might be used. If your butcher grinds the beef and stores it cold in a case or wrapped for a day or two before it goes out the door the flavor shouldn't change enough that you'd notice it after seasoning and cooking. Oxydation would have had a chance to change the color of the meat over that period, the reason why ground beef can look brown on the outside but still nice and pink when broken up. But there shouldn't be enough time for there to have a marked effect on the flavor.

If there is any perceptible flavor difference you'd probably notice it more in a burger where you're tasting the meat by itself for the most part than in something like a chili or stew. (As an aside, you don't necessarily need to grind all the meat in those anyway as they tend to be cooked long enough to soften bigger chunks of tougher cuts.)

You've basically got it. Cold oatmeal is right too.

Two main things to consider. And they're both related to control.

The act of grinding meat hugely increases the surface area. This is a problem since Bacteria and pathogens are now spread throughout the product, and have lots of surface area to grow on.

This problem gets really scary when you consider that problematic pathogens like e Coli are often spread from feces getting on meat during meat processing ( e.g. The intestines are cut open and poo gets on the meat, e coli in poo, now on meat). These are supposed to be caught and diverted, but that doesn't always happen. So they compensate with a wash e.g. Mild bleach solution). Then the meat goes to a processor who makes ground beef, in a HuGe batch, 10,000 lbs all mixed up before the machines get cleaned. The ground beef goes to distbribution and sits for a week. You go home and eat it and get food poisoning. Some people die.

That's why usda recommends cooking your burger until 165- to kill everything.

Think this doesn't happen? Go look up and see how often you get ground beef recalls. Often for e coli, in HUGE batches. Its scary.

You can buy meat thats not produced by the big boys, but you still have a potential problem.

By ensuring that the beef is ground recently, you're giving yourself a better chance of "cleaner" product.

This also dramatically affect a quality, oxidation and flavor. Fresh hamburger is 100x better within 24 hours. 4 days later and it turns grey. 7 days started turning green and sour.

Fresher is better. Exponentially better. You don't have to grind yourself, but if you're assured at its been ground that morning that helps a lot.

This affects two things A) growth of bacteria and pathogens.

I have been experimenting with different cuts on my Kitchenaid grinder. For burgers I really like chuck ground on the largest setting. The juiciness and flavor is really noticeable over anything I've had before. I believe chuck is a relatively well-used muscle part of the cow, so the meat is more flavorful and tougher. Also, the chuck steak I used had plenty of fat and tendon, so as these were distributed through the meat it made for great texture.

EDIT: I should add this is grass-fed beef, so YMMV.