Well this is an MMO.
So, presumably (Based on my experience with City of Heroes and Star Trek Online), on launch day customers who were involved with pre-order Beta will have their clients updated (unless there is a pre-launch headstart) which can be minor or major, and first time customers will need to do major downloads (the client for an MMO changes so much a disk is usually pointless by launch day) of up to several GBs.
And the week before Christmas is an odd time for a launch...especially if Santa were getting this for certain good little girls and boys. (Bet there will be a huge x-mas day hit on the servers of first time users)