One point is that of payment of a (possibly non-returnable) deposit (indicating committment) before arrival.
Although this is more than highly desirable to allow the organisers to plan the congress better, it is a significant problem as cross-border/foreign currency transfers are very expensive for small amounts of money (both for sender and recipient). It's also more difficult for electronic registration as this would need to be a separate transaction that needs to be associated with the registration.
Payment by credit card would obviate many problems, and needs to be considered/actioned early in the process - getting authorised by a card processor and sorting out a secure web site etc. is a non-trivial exercise.
Should we encourage the usage of PayPal?/WorldPay? or similar payment systems?