I want to see competitive (if not THE lowest) prices on products offered. This also includes having reasonable shipping costs. I don't want to buy a $10 or $15 dollar item only to have to pay $12 bucks or more for shipping and handling via UPS. Especially when the item could have been sent just as easily for $5 or less via USPS (and probably just as quickly too).
Also, I like to see FAST shipping/order processing. I don't even mind if it's the slowest form of shipping from the USPS. What I mean is that I want to see my items shipped within 24 hours, if not same day for in stock, non custom items.
Speaking of that, I also want to see a website with inventory that's updated daily. I don't like ordering a product (or a number of products) only to be told a day or two later that they've had the item on backorder for the last couple days, weeks, or even months.
Then of course, you've gotta have good customer service. If your customer is reasonably dissatisfied, you need to take care of them. Even if occasionally it comes at your expense. Treat customers with respect, and they'll not only stick around, but they'll recommend you even despite the occasional mistake or dissatisfaction, assuming you make it right.
That, along with what others have already said pretty much sums it up for me.