"hypothetically can't the domain name be anything and just kill the SEO on the key words?"
Yes, jobs = monster.com
"Isn't the power of a brand better than having a site that has your key words in the domain name?"
You can go either way, or have both. Shoes.com, Cooking.com, Diapers.com, Art.com, Calendars.com, Magazines.com, Golfballs.com etc. Those are well known sites with the major keywords they're targeting. Of course keywords like that will cost you an arm and a leg or aren't even available in the first place.
You pretty much can brand anything. The shorter the better, the easier to spell the better, put it with an extension the public is familiar with.