There is a lot of important factors when I chose to spend my Bitcoins online. And in general, when I buy something online.
Security, good design of store, responsive support is obvious things, so I don't even talk about it. Delivery should be fast - few days if it's in same country and not longer than month if it's overseas. Multiple delivery options is needed IMO, for example registered mail and companies like DHL. Price of goods should be reasonable and it shouldn't cost much more expensive than on other stores.
And probably most important thing for me when I want to spend Bitcoin - don't use BitPay payment proccessor. It's terrible and I'm not going to spend single Satoshi on store which use it.