I just spent $150 at Trader Joe's (we are having a party). I think that is the most I have ever spent on groceries. When I got home I noticed this post from Seth godin on why TJ's does so well. He says the 3 reasons are that TJ customers care about what they buy, have big mouths, and they are able to get great prices on private label brands. My wife and I are big TJ fans. There was one by our house in CA where we would shop every week. We were bummed when we moved to St. Louis and there was not TJ's. When we found out they were opening one a few miles from us last year, we were there early on opening day. The lines were down the aisles of the store.
I heard the guy in charge of marketing for Dierbergs say that everyone was going to TJ's to buy their $3 wine. That is a popular item, but there is more to the story. I think Dierbergs is the best traditional grocery store in the area, but they are not TJ's.
I agree with Seth's list, but I would add that you feel good when you shop at TJ's, like you are part of a club and are welcome (contrast this with Dierberg's closed door policy). The workers are friendly and will stike up conversations. This might have something to do with them getting paid more. More pay does not make better workers, but it will help to attract and retain better people. Add this friendly atmosphere to cool products with great prices and it is no wonder TJ's is doing so well.