If you are staying in the resorts, take advantage of the extra magic hour in the morning that is being offered and make that your park to visit on the first day. We really get a lot accomplished with a plan of what we want to see and knowing that extra magic hour is typically a great way to get a head start on the day.
That being said, there is just something special about walking down Main Street in the Magic Kingdom on the first day - it's what really makes us feel like we are truly in Disney World.