The reason why Parc Güell isn't just an usual garden is because of its design. It has lots of wavy forms and many other geomethrical forms, coloured glasses, ceramical scraps and it also has an sculpture of a dragon.
The fact that it's located on top of a hill and far away from the city makes this park a peaceful place to stay.
If you ever visit Barcelona, don't forget to check out this garden.
Dani.