I like the hat at MGM. I think it gives MGM some personality. And like other people said, you can't reach the water tower, so that's not a good symbol for the park. It does ruin the view of the Great Movie Ride, but you still get a good view of it if you walk thru the hat.
I also love the wand on Spaceship Earth. I though SE without the hand and wand is kinda dull. And the Leave a Legacy I think is wonderful. I didn't leave one, but I think its special and pretty. I love walking through there and looking at the pictures of people who had a magical experience throughout the years.
And I hope they get rid of all the decorations on Cinderellas Castle. I think they will ruin the symbol of WDW. I think they are beautiful for the Happiest Celebration on Earth, but they should take them down once its done.