Don't miss the outlet stores off of the property! We got some great official WDW merchandise at OUTSTANDING prices! Of course it is off season (I was buying sweatshirts in June), but they were selling in the parks for $60-$70 and I was getting them for $10-$15. THE EXACT SAME THINGS! Also, the outlet store at St. Augustine had a lot of Christmas merchandise at the time. From what I've read, these stores are hit or miss, but I'll definitely go there again, I got so many more souveniers than I could have imagined was possible for my money.
As for stores on property:
At DTD, I love their stores....They have a large selection in shops all relatively close to one another. We go there 2-3 times per trip!
like tjkraz said, if you want something resort, park, or even attraction specific your best best is in the parks, many attractions have shops adjacent to them selling merchandise for that attraction.