One of the easiest ways to irk new customers is to make them wait a long time before they receive what they paid for. For games distributed online, there's no excuse for long order delays—ideally, your customers should receive what they paid for within minutes after paying for it. Of course, to achieve this requires a fully automated order processing system, which makes you vulnerable to bogus registrations and invalid credit card numbers. Work around these problems as much as possible, but not at the expense of your true customers' time.
Be sure to explain your registration process to your customers. Tell them how long it will take, what they should do once their order is processed, and where to send e-mail or call if they feel something is not right.