I went to WDW in November 2012 and I plan to return in November 2013. Here are some mistakes I will NOT repeat on my next trip:
-Having a very annoying person accompany me on the trip
-Staying at a Moderate resort (Nothing bad about them, but Id just prefer a deluxe resort next time- namely The Wilderness Lodge)
-Carrying too large of a purse around the parks (it did fit on all the rides, but it just got annoying)
-Entering the World Showcase in Epcot to the left and walking ALL THE WAY AROUND IT to get to Japan when I couldve gotten there a lot sooner by entering on the right or taking the ferry boat. It was an incredibly long walk (especially in the hot sun and when your legs are aching really badly) and I made this mistake TWICE during the same trip!
-Riding Space Mountain (horrible)
-Not riding Rock N Roller Coaster because I received advice that I probably wouldnt like it if I didnt like Space Mountain but based on other things Ive since heard, thats not necessarily the case.
-Not spending enough time at the Animal Kingdom (I didnt get to see the tigers on the Pagani trail and lots of other things)
-Missing Illuminations at Epcot
-Not having enough pictures taken by PhotoPass photographers (they take great pictures and theres no pressure to order them if you dont want them, but better to just let them take the pictures and decide later if you want them)
-Eating at Teppan Edo (Ive had much better hibachi outside of WDW)
-Eating at Yak & Yeti (just wasnt impressed)
-Not buying a cinnamon & sugar pretzel at the popcorn stand in Epcot (I was never able to find a cinnamon & sugar pretzel at any other stand once I decided that I really did want one)
-I didn't bring any shorts because I thought it wouldn't be that hot, but one day it was and I only had jeans!
Thats all I can think of for now.