don't "save" completed quests right now, its not a good idea. The week before WOTLK goes live you can slowly fill up your quest log with completed quests, and then turn them all in when wotlk goes live. Jamming up your quest log with 20 completed quests now and having only 5 free for months isn't fun, since its much easier doing the dailies and such when you can grab 10+ quests at a time. Or if they somehow put out patches with new daily hubs, or if WOTLK is delayed, you don't want your quest log always filled up for months.