Common parts to be in stock
Decent range of stock.
Short repair times for when you need something doing.
Discounts for repeat customers.
Willingness to order stuff.

My bike shop has all of this and excellent service. Popped in today to buy a waterproof shell, extra tyre levers, chain cleaner, lube and a new seat post collar. I'd stripped the head of the bolt and needed a new one, while I was checking some other stuff they got it off and installed it for me saving me 15 minutes swearing time and didn't charge me for the part.