Starbucks in the Main Street Bakery. First, they got rid of the cinnamon buns, the CADS! Second, it kills the theme; I don't think Walt's vision of Small Town America included gourmet coffee. Now, some might say, "Hey, what about Coca Cola? That's a big ol' commercial venue, and you don't complain about that?" Well, Coke, like it or hate it, dates back to the 19th century, so yeah, it fits into an image of Turn of the Century America just as well as hot dogs, horse-drawn carriages, baseball, and the like. Caramel frappacinos, however, do not.
Now, one could make the argument that a Starbucks in Tomorrowland would not be out of place, but it doesn't belong on Main Street.