HI There!
Being a planner just like you, I have a ton of trouble waiting until the last minute to book! By booking early, I know that I will be able to get to stay at my first choice resort in my first choice room category. Once this is done, I can start planning out park days, or even think about which resort activities I want to be sure to include in our trip!
In short, the biggest advantage of booking early is peace of mind and the ability to start thinking about other aspects of your vacation. Once you are booked, you can also start making payments on the trip, if you prefer to budget this way!
As far as disadvantages, I really do not see any. If a promotion or discount comes up, these can almost always be applied to your existing reservation without a problem (providing the same room category is available at your resort). If your first choice isn't available, you can almost always change resorts/categories at that time without any trouble.
Keep an eye on the
special offers page of Disneyworld.com and MouseSavers.com to see the current discounts and call right away when you see something that will save you some money!
I have taken advantage of this many times and it always feels great to save some extra money. Enjoy your planning!