For me most important is easy integration. I don't like much complicated getaway. Second thing is fast deposit, sometimes websites required more confirmation. Even some of them delay deposit although there is more than 6 network confirmation on the bitcoin blockchain. This is disgusting for me. Another thing is secure, your website must secure by various protection in order to save deposited funds. Users should able to withdraw their fund if they don't like to buy product from you in case. Don't forget about deposit/withdrawal fees. I always avoid who ask higher deposit/withdrawals fees. Users must have to feel secure & comfortable when they will enter your site.